Contact your police precinct as soon as possible to file a report; you will need this as proof for creditors that your claim is legitimate.
Get your credit reports and scour them for anything suspicious. By law, everyone is entitled to request a free report once a year; victims of ID theft are eligible for additional reports from the major credit bureaus. For detailed info, see annualcreditreport.com.
Close any accounts that have been compromised. Ask specifically to speak to someone in the fraud department at the issuing credit agency, and keep a written log of whom you have spoken to and on what date. Follow up with a letter (to view samples, go to identitytheft.org) sent via certified mail, return receipt requested, so you have a paper trail. Documenting everything is your best defense against banks or creditors who don't believe you've been victimized.
Place an official Fraud Alert on your file. This will warn banks and other creditors to exercise caution when new accounts are opened in your name. Most alerts last for 90 days, but victims of identity theft can request an extended alert that lasts for seven years.
For information on additional steps to take, visit the FTC's identity theft site at ftc.gov/idtheft.