First, get a rulebook, so you know what's legal and what's not. I've only run in "stock" classes, so modifications were limited. Get the biggest sway bars you can find, especially in the rear. Fasten the rear sway bar as far forward as possible to reduce the arm length, effectively making the bar stiffer. All front end bushings should be replaced with a less-compliant material than stock. Buy the best shocks you can afford. There are some circle track classes that run a G-body front clip, so Bilsteins are available in the used part market. Don't go too stiff on front springs, or you'll be inducing too much front roll resistance bias. You're going to need to drift the rear if your courses are tight. Have a good alignment done, with minimum toe, maximum caster, and 0 camber. Then you can drive to the course, and add about 3/8"-1/2" shim packs to all four control arm bolts, giving you some negative camber. Front tires are more important than rears. Find the biggest tire and widest wheel you can fit up front. Rear tires can be the same size as the fronts. Most importantly, learn how to drive. Learn your car's characteristics, and how to adapt your car's characteristics to each corner. Autocrossing is a great way to sharpen your skills. Have fun!
Bill