Get a free credit report at www.annualcreditreport.com. You can get one free credit report from each of the three major credit reports, once a year. You do not have to pay for a credit report.
Make sure to check all three credit reports for any mistakes. The three reports may contain different information.
When disputing information, make sure to be specific, have documentation to prove the error, and make know your rights. If the debt is not yours, prove it. If the account was paid, show proof.