The single most important money habit to develop in your 20s

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There's a lot to learn about managing your money in your 20s.

But if you have to focus on one habit to master first, the choice is simple. The most important thing to learn is to live within your means, says certified financial planner Carolyn McClanahan.

"As soon as young people start making money, they tend to start spending it all," McClanahan, the founder of Life Planning Partners, tells CNBC.

"You have to establish the habit of spending less than you earn. Once you create the habit of saving, it stays with you for your entire life."

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Start by determining exactly how much cash is leaving your wallet. McClanahan recommends recording each and every purchase you make for a couple of months, whether that means writing expenses down in a notebook or using an app that will track your spending, such as Mint, Personal Capital, or Level Money.

If you're spending as much as or more than — you're earning, you're living paycheck to paycheck, which can easily spiral into credit card debt. That lifestyle makes it nearly impossible to build up significant savings.

"The big mistake I see young people make is assuming they can save for the future later," McClanahan says. "But before you know it, you're 50 and you don't have that much time to save for your future."

The big mistake I see young people make is assuming they can save for the future later.
Carolyn McClanahan
certified financial planner

A good rule of thumb is to "invest little bits of money all along, instead of trying to scrunch your savings at the end," she explains. The sooner you start, the better off you'll be, thanks to the power of compound interest.

That being said, if you haven't started setting aside money in your 20s, don't let that discourage you from saving and investing.

It's never too late to begin.