From the U.S. Department of Education Federal Student Aid
1) Not figuring out how much you’ll need to pay each month
As you’re trying to plan your life after graduation, it’s important that you know how much you’ll need to pay each month toward your student loans so you can budget your other expenses accordingly. To estimate what you’ll need to pay based on your income and loan debt, use the repayment estimator.
Click the link for more tips about avoiding student loan problems.