You will need to come up with the money to pay off the loan. Some places like CarMax will buy the car from you regardless of whether or not you buy from them.
However, you will need to pay them the difference above your loan payoff. Unless you have a fully loaded GT premium convertible or the Shelby, you're upside down and will need to pony up the dough.
The base model V6 would be worth about 11-12000 to a dealer. This means if you can give them the car and $7,000 in cash, then they'll take it and you can walk away. If you can't afford your 400 per month payment, you probably don't have that kind of cash laying around.
The premium trim GT (V8) Convertible would fetch only about what you owe. At least that could be a simple walk away. The Shelby could get the loan completely paid off plus about 20 grand in your pocket. Doubt that's your scenario, though.
So if you don't have the money, then your only options are to continue paying it, try to sell it for what you owe or let the repo men come. People do not generally like buying cars with existing loans on them and finding a buyer will be more difficult. Someone might come around. Probably will take a while, though. Continue making payments during this time.
You could voluntarily give the car up for repossession, though this is the worst scenario. Even if you voluntarily give the car up, it's still a repo and will still destroy any credit you have for the next seven years. The only benefit that you get with a voluntary over just waiting until they show up is that it's cheaper. You often leave the car where the lender requests with the keys, take your stuff and walk away. This results in minimal towing fees. Often times it's cheaper as you waive your rights to redemption. This speeds up the process and eliminates a lot of the added fees. For example, in the state that I am in, lenders are REQUIRED by law to hold your repossessed car for a minimum of 15 days after repossession before actual sales proceedings can begin. This is to give the debtor time to come up with the money to pay off the lender. The only thing required by law here is that the debtor must be given the time to pay off the car and all fees in full. The lender has the option of accepting less at their discretion. With voluntary repossessions, you are telling the lender you can't keep the car and can't afford it and will not try to take it back. You can often sign away the rights to the holding time. This eliminates the 15 days of storage fees of between $600 and $1000. The sales process can begin immediately after the lender has possession of the car. There may still be other storage fees for after that time along with the collection fees and the like. You will still ultimately be responsible for all of this. That will all be added into your account. The lender will request refunds of any gap insurance or extended warranties and apply this to your bill. Whatever the car sells for at auction (minus auction house fee) will be credited to your account.
You will then get a bill for any balance owed. Should you not pay this bill, further collection efforts (including suing you) may be pursued. You may or may not be able to work out some sort of payment plan with the lender.
Basically, a voluntary repo is better for you only because it saves a chunk of money. A repo is still a repo as far as other lenders and your credit report are concerned. They are reported no differently. On average, I see repo fees for VRs to be around $1500 or so. This includes all legal/admin fees added to your bill. Repo fees where the debtor just stopped paying and we had to send a repo agent to go find the car averages about $4000. This includes 30-45 days of storage, average of $500 for the repo itself, and then all the legal and admin fees to get the car turned over and sold. The debtor is legally responsible for all of these fees.