You've probably seen the TV and newspaper ads, with well-known names such as Karren Brady from The Apprentice and Dragons' Den star Theo Paphitis saying: "I'm in."

The adverts are part of an £8m government awareness campaign to promote what ministers are calling "the biggest shake-up in UK pensions for over 100 years," which starts on Monday.

Under the new "automatic enrolment" (aka auto-enrolment) regime, millions of people will be put directly into a workplace pension scheme. Both the worker and their employer pay into the scheme. You can opt out, but you'll have to do all the legwork. You will need to get an "opt-out notice" from the pension scheme, fill it in and give it to your employer – and have only a month to do it.

The government reckons that by Christmas more than half a million people will be saving in a workplace pension for the first time. But are we ready for this "truly radical social change", as it was described by pensions minister Steve Webb? This month, Scottish Widows published research showing that more than half of the UK's workers were "completely unaware" of the impending changes – though that was before the advertising blitz kicked off. Inevitably, the first time some employees will realise something is afoot is when they get their payslip and see that money has been deducted without their consent.

It's certainly a big programme: the aim is to get up to 11 million more people saving in a workplace pension. Auto-enrolment is being phased in over several years, starting with the largest employers and eventually taking in people who employ just one person. Among the first companies auto-enrolling employees will be Tesco, Sainsbury's, Asda and Morrisons.

But amid all the hullabaloo, it is worth remembering that for millions of employees, nothing is changing. If you're already in a pension at work and it meets the government's new standards, this shake-up will not affect you. The basic facts are these:

• Employers will have to enrol workers who are at least 22 but below state pension age; who earn more than a minimum amount (£8,105 a year for the current tax year – £156 a week); and are not in a workplace scheme. That means anyone on the national minimum wage and working more than 25 hours a week will be auto-enrolled.

• Workers not in that group can opt in if they wish to. If you're at least 16, but under 75, earn more than £5,564 a year and ask to be enrolled, your company will have to put you in the scheme and pay a minimum contribution, too.

• Companies will also need to enrol any workers aged 16-74 who earn less than that, and who ask to be put into the scheme, but do not need to pay contributions for them.

• If you are automatically enrolled and stay in the scheme, you and your employer pay in – and the taxman helps by giving tax relief. The total minimum contribution will start at 2% of earnings (typically 1% from the employer and 1% from the worker). By October 2018 this will have risen to 8% – at least 3% from the company, up to 4% from the employee and 1% tax relief.

People are free to say no, and many probably will because they will see this as a pay cut. But many experts are urging workers to stick with it.

"Quitting could mean losing tax breaks and employer contributions which are, in effect, 'free money'," says Joanne Segars, chief executive of the National Association of Pension Funds.

Morten Nilsson at NOW: Pensions, one of the new players aiming to grab a chunk of the auto-enrolment market, says the way to see it is: "You have to give up 1% of your salary to get 2% saved. If you opt out, you are throwing away that money your employer would have been contributing."

As to what you might get back at the end, the predictions are wide-ranging. Standard Life says a 22-year-old man earning £8,105 now would, at 67, be looking at a tax-free lump sum of £21,500 and a weekly pension of £75. It says a 50-year-old on £26,200 might expect a £13,100 lump sum and a pension of £47 a week. But these are based on some pretty optimistic assumptions. Nest (National Employment Savings Trust), a new workplace pension scheme set up by the government, estimates the figures for the 22-year-old at £4,500 and £14 respectively, and for the 50-year-old at £7,920 and £26.

Some will feel they can't afford to pay into a pension. The Department for Work and Pensions says that for many people, paying into a workplace pension is a good idea even if they have other financial commitments such as a mortgage or loan. However, it suggests you should consider opting out if your finances aren't too great: "You should make sure you can afford to meet your other commitments. If you're behind on your mortgage, rent, credit card or other debt payments, a pension might not be the right step at the moment. It's something you should come back to once your debts are more under control."

Auto-enrolment has been years in the planning, and the big test will be whether those at the sharp end – pension providers, company payroll and HR departments etc – can cope with the strain. Tom McPhail at pensions and investment firm Hargreaves Lansdown says that between 1 February and 31 May 2013, more than 6.5 million people will pass through the system while it is still in its early stages.

He reckons that some employers will simply opt for Nest as a default, without being fully aware of the other options available. Rival schemes include NOW: Pensions and The People's Pension, the latter of which is run by construction industry specialist B&CE.