What he said. The airport at Brooksville requires clubs to submit an access list a week before each event, so all autocrosses at Brooksville should require pre-regisration, and every club handles it differently.

Most other sites don't require pre-registration. You can just go (early... 8-9am) and register on-site. Though some clubs have entry limits and you might not get in that way (FAST), and some clubs charge a late entry fee if you don't pre-register (SCCA).

You might consider the FAST Novice School, it's coming up next month at SPC. Definitely a good way to get started.