A pretty much RUn what you brung class would be Street Modified, but you could also see if you fit into STS class. Either way the 06SI will nto be truly competitive, so ust go to have fun.
It usually cost 30-40 bucks.
If you really get into it you can learn to setup your car to be more competitve. But for the first few you don't need much, If you start to do it more I would say the only thing would be a helmet so you don't have to borrow a club helmet, somtimes they smell :)
THis was written by TONY, you can check all of these out.
http://www.autox4u.com/check.htm
http://www.sfrscca.org/solo2/faq/
http://www.tirerack.com/features/solo2/handbook.htm