Someone made fraudulent charges in my name. What can I do about this?

Someone made fraudulent charges in my name. What can I do about this? I got a bill for almost $18,000, I don't know yet if this is a loan, a credit card, or some other kind of debt. All I know is I didn't do it. How can I prove this? I can barely pay my own bills, let alone pay for fradulent charges someone else made in my name! If I can't prove I didn't make these charges I'll have to fill for bankruptcy. What if they do something agaist the law, will I be held responsible? Can I get a new SSI number? Is there a lawer, or anyone else that know?