The first thing you need to do is not take advice from people on internet forums who may not have done it before. It's a good way to get yourself in a potential legal or financial mess.
Liz on MSN Money offers some good advice on how to deal with this at a high level:
How to sell a car you don't own - MSN Money
Read the whole article, Page 2 has specifics on the procedure if you decide to go forward on the sell. You'll need to have some bridge financing set up unless your lien holder happens to have a local branch you can do the transaction at. In other words, you need to make some calls and do some due dilligence BEFORE you start listing your car for sale.
Bottom line, you're going to get burned on this. Based on what you said your car payment is for, I'm 100% certain you're going to be upside down, which if you don't know by now means you'll owe more than you can sell for. Also, the chances are good that you'll incur additional cost because you won't have the title (see the article I linked to above).
Unfortunately, parents don't realize they often do more harm than good by helping their kids with something they should let the kids undertake on their own. The silver lining is that you're learning from your mistakes early. A lot of adults got themselves into very similar messes over the last few years on home mortgages because they chose to follow home lenders as if they were the Pied Piper and ignore finanical reality, which is why the whole country is in the mess we're in right now. Too much debt, too much borrowing, too little forward thinking.
Good luck. And carry this lesson with you.