This is a lengthy subject, and you seem to be a bit raw. However, i don't have time right now to go into detail so I will just point out two things:
1. Pay attention to the miles and read reviews on that model for that particular year (especially reviews from people who still have the car so you can be prepared for possible problems).
2. Remember that insurance is directly tied to your age, and the type of car you buy. If you're 18 and you buy a sports car, yer gonna pay out the ass for it.