You have a few options.
You should obtain an early settlement figure from the finance company, which will let you know how much is still owing on the agreement. If you can sell the car for enough, you may be able to use the proceeds to clear the finance.
If it was a hire purchase or conditional sale agreement, you may be able to hand the vehicle back. You need to find a copy of your agreement to check this out (your finance company will send you a copy if you request it). If it is an HP or conditional sale, there will be a paragraph headed "Termination: Your Rights". In that paragraph will be an amount equal to half of all the payments added together.
If you have already paid at least this amount, you can hand the vehicle back with nothing further to pay (as long as you have taken reasonable care of it).
If you haven't yet paid this amount, you can still hand the car back, but you will have to make up the difference between what you've already paid and the amount specified under the Termination clause.
This only applies to HP and conditional sale agreements, so if you got a personal loan to buy the vehicle, you cannot use this voluntary termination method (but you can still sell the vehicle and use the proceeds to pay the early settlement figure).