Read the rules and decide which class you want to run in.
Some of the changes that you make will affect which class you will run with.

Do all of these but I am still going to list them.

1st Upgrade the bushings on your car. Replace worn out rods/arms
2nd Bleed the entire brake system and refill with new brake fluid.
3rd Upgrade brake lines and upgrade your brake pads to an aggressive street pad, like Hawk HPS.
4th Get a race seat and soft compound tires. These will make a world of difference.
5th SEAT TIME, SEAT TIME, SEAT TIME