Thursday afternoon, San José, California. Justin Bieber sits in a dressing room that's usually home to the San José Sharks hockey team. But there are 23 Sharks and only one of him – a tiny boy in a huge room, empty but for a sofa, a documentary-maker pointing a camera at me (Jon M Chu, director of Step Up 3D, now making Justin Bieber Never Say Never 3D), his press man Mike, and his Xbox. He has, I notice, chosen for his Xbox avatar a small black girl wearing a plaid skirt.

Jon M Chu's 3D camera is capturing a circumspect, anxious me. My anxiety is due to the fact that Justin – being only 16 and by far the biggest teen star in the world, probably the biggest since Michael Jackson – tends to rattle off overly polished maxims in interviews, such as these from his newly published autobiography, First Step 2 Forever: My Story:

"Every one of my fans is so special to me... It all happened because of you. I wake up knowing I have the best fans in the world… My team is my family and they all deserve their time to shine too…"

This guardedness is understandable given the millions of anti-Biebers out there, ready to pounce on any misstep, although they're a drop in the ocean compared with the billions of Beliebers in the world, a Belieber being – according to the Urban Dictionary – "A person who loves Justin Bieber & beliebes in everything that he can do". According to Twitter, 3% of all their traffic is Belieber-related, with servers all over the world dedicated to Justin and his fans. In July he overtook Lady Gaga as the most searched-for person on the web.

I haven't got long alone with Justin and I'm worried that there won't be time to burrow beneath the platitudes and find some (with any luck) fascinating darkness.

"Which funny YouTube videos have you been watching lately?" I ask him.

"There's one called Scarlet Takes A Tumble that's really funny," he says. "This woman's on a table and she's singing and all of a sudden she stands on the edge and flips over and falls. It's really funny."

"What about Charlie Bit My Finger?" I ask him.

"Not really that funny," he says. "You think Charlie Bit My Finger is funny?"

"I do," I say.

Justin shrugs. "To Americans and Canadians it's just funny because of their English accents." He pauses. "There's a video called Arab Screaming that's really funny. It's an Arabian guy who starts screaming. It's just hilarious. You should see it. Go."

I ask Justin if he ever looks at his own YouTube videos. He says while he understands the perils of Googling himself, he does sometimes read the comments. " 'You're so stupid', 'Your song sucks', I even get, 'You're gay' for no apparent reason. What's the point of that? But then I remember there's so many people who like my videos who don't even comment. When I like a video I don't waste my time commenting. But people who hate you – they're going to take time to hate you."

He is somewhere in the midst of an 85-date tour. A row of buses is parked back in the loading bay. They drive in convoy through the night from city to city, carrying Justin and a vast army of grown-ups. I see them backstage: anxious-looking men wearing suits and holding clipboards, grizzled roadies. Earlier I watched Justin weave in and out of them on his Segway.

"Does all the travel make you feel lost?" I ask.

"You're so far away," he nods, "and you start feeling like you're a robot. When I'm overseas the schedule is always crazy and then there's the time change and you're not even yourself. It's weird."

"Do you ever feel wistful for the days before you were famous?" I ask.

At this Justin looks as if it's all getting too introspective. "I'm a regular person," he says hurriedly. "I'm living my dream and I'm just enjoying every minute of it."

It was YouTube that made him famous. His mother, Pattie, who had him when she was 18, raised him alone in the small town of Stratford, Ontario, Canada.

"My mom wasn't the greatest person," Justin says. "I mean, she was a good person, but she made mistakes. She drank. She probably did drugs and stuff, and she told me about it because she said she did enough bad stuff for the both of us. I don't need to do it because she already did it. The fact is, she changed her life around because of me. When I was born she quit smoking, she quit drinking." He pauses. "She did that all for me."

His father, Jeremy, left home when Justin was three, although if you look hard enough on the internet you'll find pictures of him – a tattooed bodybuilder calling himself Lord Rauhl, whose bio reads, "My life is my son. He is 9 years old and is the most talented person I know. He's a 'looker' too (just like his dad)!"

And Justin turned out to indeed have unexpected talents: "I can do a Rubik's Cube in a minute and a half," he says.

"Whatever state it's in?" I ask.

"Yeah."

"Are you a genius?" I ask.

"I wouldn't say a genius, but I can do a Rubik's Cube," he says. "And sudoku puzzles."

And furthermore, he had an effortlessly cherubic singing voice. When he was 13 he started busking to try to raise enough money to visit DisneyWorld. Overwhelmed passersby filmed him and put the videos on YouTube. He entered local talent contests and his mother put those videos on, too. They were noticed, first by a manager named Scooter Braun, then by Usher and Justin Timberlake, who got into a bidding war (Usher won), and then by zillions of teenage girls, all at once, all over the world. Most stars win over countries gradually, one by one. Justin instantly became massively famous everywhere. So they pulled him out of school and he began the life he now has – home-schooled in the tour van, always on the move.

"Singers aren't supposed to eat dairy before a show but we all know I'm a rule breaker."

First Step 2 Forever: My Story

A few months ago, while visiting a Sydney TV show called Sunrise, Justin was being led by the floor manager to the couch. He put his hand on Justin's back.

"Don't ever fucking touch me again," Justin reportedly growled at him.

"Oh, he tells us that all the time," Justin's sound man murmured to the startled floor manager.

A few days after the Sunrise incident, Justin was in New Zealand, being interviewed by a presenter with a strong Kiwi accent. He asked Justin if Bieber was the German word for basketball.

"German?" Justin asked.

"German," said the interviewer. Justin looked blank-faced. "German," said the presenter. "You know? German."

"I don't know what that means," said Justin.

"Here," said the interviewer, showing him the word German written on his card.

"I don't know what that means," said Justin. "We don't say that in America."

They were a disastrous few days – Justin's very own Scarlet Takes A Tumble. "Couldn't he squeeze in a few minutes to learn what 'German' means?" wrote the Huffington Post. "The Beebs doesn't know that the word 'German' exists."

Soon after a video emerged, filmed in Germany months earlier, in which Justin counted to 10 in German and spoke lovingly about his German great-grandfather.

"I couldn't understand what the guy was saying," explains Justin now. "I know what German is. Obviously." He pauses. "It sounded like he was saying Jewman."

"You thought he was asking you what the Jewman word for basketball was?" I ask.

"I didn't know what he was saying." Justin pauses. "Mike?" Justin turns to his press man, sitting in the far corner. "You were there."

"The way he posed the question was confusing," Mike agrees. "He was trying to be funny."

"But it's not even German for basketball," says Justin.

"It was a joke," says Mike.

"It was weird and I didn't get it," says Justin.

"By the way," I say, "the Jewman word for basketball is 'The game we don't play well'."

"Are you Jewish?" asks Justin.

"Yes," I say.

"Nice," he says.

He recites the first line of the Shema – the Jewish morning and evening prayer – getting it syllable perfect: "Shema Yisrael Adonai Eloheinu Adonai 'Ehad." He pauses. "Christianity was based off of Jesus being a Jew," he says. "I respect it." Justin is a practising Christian. "I pray all the time," he says. "I pray two to three times a day. When I wake up I thank him for my blessings. I thank him for putting me in this position. And at the end of the day I get out my Bible. At home-school my tutor is Christian, so we go over Bible verses. It's something that keeps me grounded."

"When you meet atheists, do you think they'd be better off if they were Christian?" I ask.

"They're definitely missing out," he says.

"What's it like being home-schooled?" I ask.

"I only have to do three hours a day, which is good," he says. "I drift off. I definitely drift off. So I'm better one-on-one."

But paradoxically, he says, he is also a perfectionist. In fact, his perfectionism is a problem: "I'm way too hard on myself. I always want to be better."

"Do you beat yourself up too much?" I ask.

"A bit too much," he says. "But lately I've been trying to get better over it."

Drifting off; outbursts of anger; being a perfectionist... "Do you have ADD?" I ask him.

"I…" Justin says. "I have a small case of ADD."

"How does it manifest itself?" I ask.

"If I don't understand something, and I'm bored, I don't pay attention," he says, "so my teacher has to really make it fun for me. Every hour he has to give me a five- to 10-minute break. But after the break I'll be back into it. I'll be good."

"Have you actually been diagnosed with ADD?" I ask.

"No," he says. "It's self-claimed."

"I've diagnosed myself with generalised anxiety disorder," I say.

"Nice," he says.

"Do you get anxious?" I ask.

"Yeah, I get anxiety sometimes," he says. "Mostly when people are pulling me in 20 different directions. That's when I get it. I'm, 'Let me just breathe for a second'."

"If I were you I'd constantly be telling people not to fucking touch me," I say, "because you're forever being harnessed and prodded."

"It's tough," Justin says.

He denies, by the way, telling the Sydney floor manager not to "fucking" touch him – "Hearing adults spread lies and rumours is part of the job, I guess," he tweeted at the time – but he admits losing his temper with him. "There are so many people telling you what to do," he says. "I work really hard, and I'm tired, and I have bad days." He pauses. "Everyone has bad days."

He tells me a story about before he was famous. He approached the rapper Twista for an autograph: "He was, 'Not right now.' I was, like, 'Dang.' But now I'm in that situation I understand where he was coming from. You've got so much going on. Like if I'm eating. I don't want to be bothered when I'm eating. That's my only downtime. People don't understand. They think it's all fun. And everything's great. But it's hard work." He falls silent. Then he adds, with some vehemence: "It's harder than anyone thinks."

"The huge steel-framed hot air balloon basket is designed to fly me out over the crowd, dipping not quite low enough for them to touch, but close enough for me to see all those beautiful faces."

First Step 2 Forever: My Story

Justin writes in his autobiography that his favourite topic of conversation is, "Girls, girls, girls, girls, girls, girls, GIRLS." I tell him that I am prepared to ask him about girls, but self-consciously so, for I am a 43-year-old man and I think it will seem creepy.

"You can ask me about girls," says Justin, reassuringly. "It's all good. I like girls. Girls are awesome. Yeah."

"How will you ever find a girlfriend who won't just spend the whole time thinking, 'It's Justin fucking Bieber'?" I ask.

"That's what's hard about this," he says. "There are so many girls who would just do anything for me because of my status." He pauses. "Someone told me it's great to be with somebody who has just as much to lose as you do."

"So you'll have to go out with someone famous?" I ask.

"Yeah. That's probably a good idea. Because I can never date somebody who's so in love with me that she would do anything for me."

He shrugs as if to apologise for the downbeat, pensive tenor of the interview. He says I should be glad I've not got him on a bad day.

"Mike knows my bad days," he says.

"Act out what you're like on a bad day," I say.

"OK. Ask me a question," he says.

"What do you think of that sofa?" I say.

Justin gives me a look of boredom tinged with withering hatred: "I like it," he mutters. "Next question." He gazes off into the distance.

A chill runs through me. "Thank God you're not having a bad day today," I say.

"I'll be zoning off," he says. "I'll be over there. I'll not be focused on you. Good days I'll be…" Justin leans over until he's an inch from my face. He gives me a vast and petrifying smile.

Outside in the lobby, hundreds of competition winners are readying themselves for their meet and greet with Justin. Mike the PR explains that the concept has changed over the decades. In the old days it might have taken the form of a drinks party. But now it's a long line of people, ushered, four at a time, into a room in which Justin is standing. They say hello, get their photographs taken, and are ushered quickly out again.

And it is never-ending. Fan after fan after fan. Justin says hi. They shriek and look as if they're going to pass out. They have their pictures taken with him. They're gone. Each encounter lasts perhaps 30 seconds in total. Many have brought letters, which they hand to Justin, who hands them on to someone else, who hands them on to someone else, who puts them on a nearby counter. What happens to them after that, I don't know. I have a quick read of a few of them:

"So I'm sitting here trying to write this letter to you. I didn't think it would be this hard… I'm really just this short person…

"Call me: 510-502…"

One girl hands Justin a letter and then seems overwhelmed with doubt and suspicion.

"Read it," she barks at him. Then she turns to me. "Watch him as he reads it," she yells.

Nearby, there's a larger-than-life-size cardboard cutout of Justin. The real Justin wanders over to it. "Who are you looking at, buddy?" he mutters. He punches it in the face.

A few minutes later and another girl's moment with Justin happens to coincide with him being briefly distracted. "Jesus!" she hollers as she's shepherded away. "He didn't say hi or nothing."

But Justin is already posing for another photograph.

Justin Bieber's My Worlds: The Collection is out on Mercury Records on 22 November.