Hi N.,
The first thing I would do is contact your ex and find out why he's not making payments, where is the car, etc. Keep a record of everything, date and time you talked to him, copies of writen correspondence. Maybe you can work something out.
Contacting a lawyer would be your best bet, but I'm guessing you don't have the money for that. So...
Whose name is on the regisration with DMV? If it's your name only, your in luck. Contact your local police. However, if it's both your names or just his, he has the legal right to possession.
Did you have type of writen or verbal agreement with your ex? If not, your chances of recovering anything in court are not very good. Since you signed, your responsible. If there was an agreement between the two of you, small claims court is probably a good way to go.
The next thing I would do is contact the company financing the car. Explain your situation. Tell them you are not in possession of the car, and can not afford the payments. In most cases they will work with you. (A word of caution, in rare cases they won't work with you and try to bully you, don't let them do this.)
The person or company who holds the title is the legal owner of the car. Since payments are still owed this would be the company that financed the car. They have the legal right to reposess the car since they are the owners. This of course will destroy your credit, but keep in mind it's already damaged and this will prevent you from owing additional payments.
Remember to keep records of everything, so if this does wind up in court, you'll be prepared.
Once it's all resolved, write letters to all 3 credit bureaus explaining what happened.
Good luck.