I'm not going to disagree, but whatever you autocross shouldn't be certain things like a car set up for drag racing or making way too much power for the platform. While there are smaller clubs and such that might let you run a truck I wouldn't suggest that.
Something that will teach you good habits, a car in good condition mechanically, but most important to me is a car that you will enjoy outside of autocross. Chances are autocross will be fun, but not something you will take very seriously, so get a car you will enjoy driving otherwise. This was were I screwed up buying a Mazdaspeed, I hated that car, but I loved my 91 Miata. So make sure you like whatever it is.