If your car is completely stock I would say just start off with some off the shelf Koni Yellows and some Kumho VictorRacer tires on stock wheels.

Modding your car up too much will bump you into more difficult and sometimes more competitive classes.

I say save your money for an Evolution School http://www.autocross.com/evolution The money spent on that will shave off more time than mods to your car.

After becoming an instructor last year I've been trying to tell this to all my students. They come and ask me why they only dropped .1-.5 seconds (compared with others in their class) with their new $1000 suspension. Then I jump in the car with them and try to fix the nut behind the wheel and then they shave off 4-8 seconds