You will need a helmet. It will need to have a Snell rating. This is the case for cars that run quicker than 13.99. You also need to wear long pants. That is about it as far as the rules go for the class a stock or lightly modded GTO runs in.
Here is more than you bargained for, some advice. Get to the track real early, before anyone starts to run. this will give you the chance to walk the staging lanes and the strip before there are cars there. If you can get someone that has been to the track to walk it with you. Have them explain the basics. Also it is best if you sit and watch some cars go and have someone that is knowledgable explain what is going on.
Some basics are: never speed in the pits. Watch the NHRA/IHRA officials and follow their instructions. Never do a burnout while the cars are still staged ahead of you. On the tree there are staging lights, you need to pull forward til the bottom pair are lit. There are 3 yellows, a green and a red. When you think you see the bottom yellow light you go focus on it. If you have the outside lane, never cut in front of a racer to get to the turnout, the race is to the finish line, not the first guy back to the pits.
Drag racing is a lot like golf. If you try to kill your tee shot people laugh at the results. In drag racing if you try to kill it off the line you end up overpowering the tires and doing nothing. Just like golf clubs have a sweet spot, there is a sweet spot for launching. If you overpower a drive it doesn't give you the result you want and the guy that went too easy beats your drive. Same in drag racing. It's better to go too easy than past the limit.
Practice is key, just like any sport. Don't listen to just anyone at the track, and everyone has opinions. Watch the guys that seem to run the fastest and ask them for advice. Most drag racers like to help others.
The most important thing to remeber is it's just for fun. Most people that golf aren't John Daly or Tiger Woods. Most drag racers aren't John Force.