If you're talking about road courses, it depends on the organizer and if you want to be the first one there to have the prime parking spot while you wait for your sessions.

Groups like NASA does tech inspections where a tech actually check your car. Most others just require you to do a self-inspection where you just fill out a form and give to someone when you check-in prior to the first session of the day. Since you do a self-inspection you can pretty much show up anytime in the day.

I highly recommend Streets of Willow. It's a fun track, very technical and is more suited to cars with lower HP.

And once you've got bitten by the track bug, i suggest trying out Laguna Seca up in Monterey. Expensive but at least you can say the "corkscrew" is a piece of cake. :-)