Contact the fraud departments of any one of the three major credit bureaus to place a fraud alert on your credit file. The fraud alert requests creditors to contact you before opening any new accounts or making any changes to your existing accounts. As soon as the credit bureau confirms your fraud alert, the other two credit bureaus will be automatically notified to place fraud alerts, and all three credit reports will be sent to you free of charge.

Close the accounts that you know or believe have been tampered with or opened fraudulently. Use the ID Theft Affidavit when disputing new unauthorized accounts.
File a police report. Get a copy of the report to submit to your creditors and others that may require proof of the crime. Instructions for completing the ID Theft Affidavit

Finally, File your complaint with the FTC. The FTC maintains a database of identity theft cases used by law enforcement agencies for investigations. Filing a complaint also helps us learn more about identity theft and the problems victims are having so that we can better assist you.

Information courtesy of http://www.consumer.gov