On Cars

I have never owned my own car. Living in a giant metropolitan area as I do, this is becoming more and more problematic. While my roommate has stated on several occasions that she's more than happy to cart my butt around should I need it, that limits my transportation to bus schedules and her availability. Plus, I feel like a huge mooch, even when I do chip in for gas.

I would like to purchase a car. There are a couple of problems with this:

1) My credit sucks, due to some credit card debt from years and years ago.
2) I don't know anything about cars.
3) I have a lot of expenses in the upcoming year.
4) Insurance in Arizona is insanely expensive.

Any thoughts, ideas? Any help at all would be much appreciated.