Self-made millionaire David Bach made the biggest money mistake of his life on the first day of his freshman year at USC, when credit card companies were tabling in front of his dorm.
Representatives were giving away bike locks and sweatshirts to students who signed on and, "before I knew it, I had three credit cards," Bach tells CNBC Make It.
"I did everything wrong," he continues. "I used those cards — I told myself I wouldn't, only in case of emergency purposes — but I used them. I maxed them out. I started to make only minimum payments."
By sophomore year, he was $5,000 in debt. "It was the worst financial mistake of my life. And it was painful."
Bach, who has since written bestsellers including "The Automatic Millionaire" and "Smart Women Finish Rich," still advises students to open their first credit card in college. That's because opening a credit card can help you establish good credit, which will allow you to make larger purchases in the future, such as a car or home.
"Get the credit card, because it's great to help you build your credit score early," he says. But use it responsibly.
"You don't have to use a credit card a lot to build your credit score — you just have to pay your credit card bills on time and pay those cards off in full every month."
Bach isn't alone in learning this lesson the hard way: Americans owe a collective $1 trillion in revolving credit card debt.
If you're already in debt, there are two popular ways to get a handle on it: the avalanche method and the snowball method. The first prioritizes paying down the debts with the highest interest rates first, while the second focuses on knocking out the smallest debts first.
You can also read up on some habits to give up if you want to get out of credit card debt.
Don't miss: Money expert David Bach: Use this formula to figure out how much you should have saved at every age
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