Be careful about going crazy with upgrades. They can very quickly move into a class where the car is uncompetitive. All you really need is some better pads front and rear and good brake fluid.

While you're still learning how to autocross, run good street tires. Falken Azenis RT-615s or Bridgestone RE01Rs are excellent. The street tires have more progressive break away characteristics whereas a tire like a Toyo R-888 or Hoosier A6 will have a narrow envelope before losing grip.

Most important, though, is having fun.