Getting rich can be straightforward. Mostly, it requires making smart choices, like committing to paying yourself first and setting aside at least 10% of your pretax income in a retirement account, says financial adviser and self-made millionaire David Bach.

“With this in mind, it shouldn’t be hard to figure out the single biggest investment mistake you can make: Not using your [retirement] plan and not maxing it out,” Bach writes in “The Automatic Millionaire.”

The good news is, it’s easy to fund a retirement plan.

If your company offers a 401(k) plan, start there. You’ll get large tax advantages, the money is automatically taken from your paychecks before you have the chance to spend it, and sometimes your employer will match your contributions up to a certain amount, which is essentially free money. The maximum amount you can contribute to a 401(k) in 2017 is $18,000 a year, or $24,000 if you’re 50 or older.

If your company doesn’t offer a 401(k), you can fund a traditional IRA, Roth IRA, or myRA, which all offer tax breaks and are designed specifically for retirement.

With a traditional IRA, you contribute pre-tax dollars — up to $5,500 a year, or $6,500 if you’re 50 or older — and pay taxes on your contributions and earnings when you withdraw the money.

With a Roth IRA, contributions are taxed when they’re made, so you get to withdraw the contributions and earnings tax-free. The same contribution limit applies ($5,500 a year, or $6,500 if you’re 50 or older) and there’s an income cap: If you’re single, you have to make less than $117,000 to fund a Roth and if you’re married, you have to make less than $184,000.

Finally, myRA, or “my retirement account,” is a relatively new retirement savings plan launched by the U.S. government. Like a Roth, you contribute after-tax earnings that can be withdrawn tax-free. The income cap is $132,000 is you’re single or $194,000 if you’re married and filing jointly, and you can contribute up to $5,500 a year, or $6,500 if you’re 50 or older.

The biggest limitation is that, once you have $15,000 in your myRA, you have to roll the balance over into a regular, private IRA.

Whatever retirement savings account you choose, the most important step is to open one and start setting aside as much as possible, preferably 10% or more of your pre-tax income, Bach says. Then, make your contributions automatic, by getting your employer to do a payroll deduction or by having your money sent from your checking account to your IRA.

Finally, aim to max out your plan. If you can’t max it out yet, get in the habit of increasing your contribution on a consistent basis.

Most importantly, start today, if you haven’t already begun, Bach says: “Maximize your retirement contribution now. Take just this one action and you will have changed your financial future for the better — guaranteed.”