Paying for cars monthly?

I am 17 and looking to buy a car on a pay monthly basis I have seen a few in the local newspaper but I need more choice. I would be grateful if you could get me websites were you can buy cars on a pay monthly basis used or new. My budget would be up to �100 p/mth. I would also like to know how long the it takes to pay off the car is it similar to a phone contract? were it takes 18 or 12 months. Thanks MK