Take it you want to do track days?
Brakes and tyres will make the biggest immediate difference I think.
If your on a really tight budget talk to
@George@RTR_Parts about the brakes. Probably hc front disks and ds2500/3000 pads, Leave the rears standard, braided lines and replace the fluid with performance stuff. I use ATE stuff which is cheap as chips and I hammer my car and don't have a problem.
You can find some decent second hand tyres very cheap on here/eBay etc. Be warned about track tyres on the road though. Doesn't take much to make them illegal and some are lethal in the wet. There are some that are half way between the two though.
Other than that make sure your filters and fluids are all good. General maintenance is kept up and go and have a blast.