Agree with all of the above...should have started with a school, but ok...let's get you prepped.
First...make damn sure you are placed in the slowest group. Hopefully they spend a bit of extra time talking to you folks in the drivers' meeting. Pay attention.
Check and see if either the event or the track have a checklist/rules/regulations...read them. Read them again. Start learning the flags.
Go through your car...everything that isn't bolted on is out (trunk, under/behind seats, etc.).
Get all over and under your car and tighten everything. Now use up some nervous energy and do it again.
Check your tires for wear/rubbing (asspecially the inside against suspension/frame).
Check your helmet...the proper Snell rating is required. No motorcycle helmets. They might require face protection in an open car. Check condition of your belts...make sure your seat bolts are tight (yup...it happens).
Rig up a puke tank if you don't have one for the coolant. Some tracks allow a soda bottle...others want something a bit more comforting for the track's sake.
If you're using floor mats and they aren't attached to keep from sliding...pull them out.
I agree with the above...many tracks won't let you have a guest if there's no additional rollbar in an open car.
As you drive around the next few days, start exercising your situational awareness of the cars and scene around you.
Besides pumping up the tires...check them for any rocks, etc.
You need to keep it at a comfortable pace and don't get cute. Everyone has their first day or two out there and nobody expects much from you except the ability not to phuk up anyone else's day. Those first few laps will either scare the hell out of you, or you will be in love with it. Either way...let the car warm a bit...
oil, tires, etc. Get comfortable driving your car on the track. Watch how others are taking corners.
Do not get on someone's ass and follow them just because they look like they know what they are doing. If they're in the slow class...they don't. It's like seeing someone you know in a porn shop.
You'll be ok...have fun, and promise us you will sign up for a class in the near future because you're gonna love it.