That's an EASY one! Get the rule book from the sanctionong body (SCCA?) and study it!
The first thing you have to decide is which class you want to run in. You MUST familiarize yourself with the rules in order to do that.
There is a "bone stock" class that prohibits almost all modifications (you even have to run the stock seatbelts) but still allows brake pad and tire changes, and aligment and setup changes to maximize the handling of your car. Most racers buy a second set of wheels on which they mount a set of DOT-legal race tires (which would wear out on the street in 2000 miles.) This class can still be a little expensive because of the frequent brake and tire expenditures, but is the cheapest class to run.
The next class up is "street prepared" where springs, sway bars, shocks, wheels, etc. are fair game, but you pretty much need to change ALL of those things to be competetive. You MUST read the rule book in order to know what is legal and what isn't.
Good luck and have fun.
Gary