Call the police first. Every step of the way, you will be asked for documentation of the forgery, and the police report is it. Retain a copy of the report even after the forged property is recovered. You may need it to refute any other charges that may arise later.
Contact your bank or credit card company. Change your accounts and close the forged ones. With a check, request that "forgery" is stamped on them to remove yourself from responsibility for the charges. Act immediately with your financial institution so that you can cut off all activity on your account, and possibly catch the criminal in the act.
Go online or call all three credit bureaus. They can put out a fraud alert that will stop the forger from opening more accounts in your name. Review your credit report on a regular basis thereafter to ensure that no other fraudulent activities have been made on your account.