First obtain your credit report and find any inaccuracies at AnnualCreditReport.com. You must then send a letter to one of the three big credit reporting companies requesting that the inaccuracies be corrected. You will have to include supporting documents to show that you are right. Send copies, not originals. The credit reporting companies are required by law to investigate your claim and correct it if there is an error.
If someone is reporting inaccurate information to the credit reporting company, you may have to write to them and request that they correct the inaccurate report. If something important is left out of your credit report, do the same thing, write to one of the credit reporting companies.
The Federal Trade Commission has put together a set of step-by-step instructions on how to do the above. It includes the names and address of the credit reporting companies, sample letters to use, and other helpful and pertinant information. Go to this website: http://www.ftc.gov/bcp/edu/pubs/consumer/credit/cre21.shtm