There are loaner helmets and they are free to use. Think of a bowling shoe, and think of putting it on your head. 
You’ll want to bring a little portable air compressor to adjust your tire pressures. If you don’t have one you can usually find one to borrow at the event. A piece of white chalk or white shoe polish is also helpful because you can mark your sidewalls and see how far the tire is rolling over to help decide if you add or take away pressure.
Michaels sells magnetic sheet that you can cut out numbers from, but it was pretty thin and I had some trouble with the class letters flying off (they were smaller than the numbers). Someone bought some pretty thick magnetic sheet last year but I forgot where they got it. Home Depot maybe? You can always just print your number and class out in Photoshop and tape it to your car or inside rear window. Number + class must be on both sides of the car, and big enough to be read at speed from a distance. Normally there is a box of paper numbers at registration but it’s easier to make them at home.
Sunscreen a must. Drink plenty of fluids. A folding chair is nice for when you’re not working or running. (That reminds me, I have to see if my blue one is in the truck from last year).
Don’t be afraid to ask questions. I’m usually there pretty early just wandering around so look for my red GTO.