That doesn't add up. Why does it have to be cheap? If its because you don't have a lot of money - then you wont have the funds to upgrade it and keep it running. What kind of car are you looking for? Something to take offroad, passenger vehicled (and how many people need to be in there), long distance driving?
Why does it need to be upgraded? If you don't know what kind of car you want yet, I doubt you know how you want it to look in the end. If your not familiar with the performance aspects of a car, then those enhancements are much harder to judge and often expensive.
If your just a kid (and that is my guess), learn how to drive responsibly first and decide what you'll get a feel for the style of car you'll want from there. If you want a first car to learn in then a civic, gulf or neon would be more then adequate to get started with.