Hey, high school graduate of 2015! Congratulations on your graduation.

You don’t know me, but I was you — about 30 years ago.

There’s a whole bunch of stuff that I wish someone had told me on the day I graduated. It would have been really, really useful.

But nobody did. So I had to learn it the hard way.

I want to save you all the trouble. So if you want to have a much easier time over the next 10 or 20 years, ponder these 25 suggestions.

1. Most of what you’ve just learned in high school is completely useless and you will never use it again. That time spent drawing stamens or reading Edmund Spenser’s “Faerie Queene”? Totally wasted. And you will never use calculus again.

2. Just because you succeeded in high school doesn’t mean you’ll succeed in life. It’s totally different. In a few years nobody will care what team you played on or what your GPA was. Sorry, but you’ve got to start all over again.

3. Don’t confuse your studies with an education. Time spent getting an A-plus in college, as opposed to an A-minus, is probably wasted. But keep learning all your life. Ask questions. Listen to people. Take classes. And keep reading. Always have a good book on hand.

4. Truer words were never said: “Do a job you love, and you’ll never work a day in your life.”

5. Don’t covet “stuff” and don’t waste money on it. If I had back all the money I spent on junk in my 20s and 30s, I’d be in fat city. Throw something out whenever you buy something new. Clear out your closets every six months and throw out everything you don’t absolutely love.

6. Dress for other people, not for yourself. It’s counter-intuitive, but true: Other people are the ones who have to look at your clothes. It doesn’t matter what colors or styles you happen to like. Find out what looks good on you, then wear that.

7. Potential employers don’t want to hire young people because they don’t know if you’ll turn up on time, dress properly and act professionally around the office. If need be, offer to work for free to show them you can.

8. Most people are brand junkies. That includes most potential employers. So get the best blue-chip company names on your resume that you can.

9. When you are starting out in work and you don’t know anything, at least show the right attitude. You’d be amazed at how far that can get you.

10. Take a Myers-Briggs personality test early on. The sooner you understand your own psychological strengths and weaknesses, the better you can work on them (and with them).

11. Read “Reinventing Your Life,” the classic textbook about cognitive behavioral therapy. Learn about your own cognitive errors, and how they are holding you back.

12. Keep a to-do list every day.

13. Read “The Age of Propaganda” and learn early on just how you’re being manipulated by spin-doctors and marketers.

14. Read “How to Lie With Statistics” and “Say It With Charts” to understand how you can use statistics, and how you can protect yourself when others misuse them.

15. Don’t take a year off before going to college. But do take at least two years between college and grad school, and do a real job.

16. Learn early on about professional project-management, problem-solving and (especially) hypothesis-driven analysis. For example, read “The McKinsey Way” or a research paper. They’re not just for management consultants. These simple techniques will save you a ton of time and pain down the road.

17. Don’t evaluate people on what they say, but on what they do. It always turns out to tell the true story.

18. Never develop a taste for luxuries. Then you’ll never need or miss them. The wise can live better than the greedy on a fraction of the cost.

19. Don’t become a tech junkie. I’ve lost track of how much time I’ve wasted either dealing with tech problems or surfing the web. I wish I’d never had a laptop in grad school. I would have been so much better off taking notes on paper, like a Luddite.

20. Read “The Pyramid Principle.” It’s a terrific guide to structuring your thoughts and written communications, simply and effectively.

21. Three simple rules will explain 99% of human behavior. 1: Most people don’t think. 2: Some people are just jerks. 3: Everyone is selling something.

22. Move to where the jobs are.

23. Stretch yourself beyond your comfort zone, in work and in life — but only in stages. Don’t try for too much at once.

24. Never neglect your mental health. Your own happiness should be Job 1. Without it, you’ll struggle at everything else.

25. You won’t actually find out who your friends are until you hit trouble. And then you’ll be surprised.