New driver I'm 18 and need a car. Help?

I just fot my license 5 months agoI'm 18 and I don't live with my parents. I live with my grandma. I have a job which I get $500 a week. The only bills I have to pay is my phone bill which is $60 so that's not a problem. Anyways.. Starting college in 2013 fall semester and I need a car to drive. Well I have no credit or bad credit so. Since I make 500 a week I should be able to find a good car. I was thinking more like years between 2004-2007 sedan type of car. Fusion, Pontiac, or any good car that's automatic.

Ill be looking at a auction and Craigslist for a car. So if I buy from either both on a used car 2004-2007 what would my monthly payment be? Insurance? I'm really confused about this and I want a full explanation on what I'm missing here. Remember I'm pretty much on my own and I want to do everything by myself. Will making $500 a week help me with car payments and everything? Just Someone explain to me. Thank you