All mods will put you in different classes. SCCA is the big time, but you have local events as well.
check out www.auto-x.com for a list of some classes and where mods will land you. But you can always do novice till you place first then you out. But its good exp and seat time to not get first and learn your car and get the feel for how it handles as is.
Tires/suspension mods will move you classes quicker than anything.
Hope this is of some help