For tires, you should be jacking up the pressure about 5~10psi over what the door sill says. Tires that are not dedicated for Autocross are going to have more flexible sidewalls, higher pressures will help prevent them from rolling over. You can use some chalk to see how far they roll over on each run. If they are running onto the sidewall, add more pressure. The pressures do go up after you do a run, especially after your first one. You can just bleed them off a bit to get them back to an optimal pressure you are happy with.
Ensure you have enough food and water for the day, bring sunscreen for sure and if you have a golf umbrella that is awesome for hot days. Also bring a tarp if there is a possibility of rain to cover your gear.
If you club allows it, try to get a ride long before you go out. Also try to get somebody with experience to jump in with you as well, it helps a lot to have somebody knowledgeable along for the ride.
Your headrest will be a PITA when you put on a helmet unless you flip it backwards. To do that, use the key inside your FOB to depress the little button on the opposite side and it will slide right out. You can put it in backwards after that.
Mostly just try to have fun with it. Don't get too upset about missing a cone or gate. If it happens, just remember that you paid to be out there, and nobody will give you grief for continuing on with your run. Also don't be afraid of the cones, you have to get close to them and learning where the edges of your car is usually means you have to knock a few of them down! If they do leave a black rubber mark on your car you can just clean it off with a Mr. Clean magic eraser later.
Have fun!