Hey Brittny! I am SO pumped for you! Going to cheer tryouts has certainly been one of the best choices of my life- I've made SO many new friends, I get to mix in with girls from other grades, and we get to travel and wear super cute clothes! XD
Woah! Ik you said you don't know a lot, but a little IS a lot! I went into tryouts with literally ZERO idea what I was to do, ahaha! Gymnastics is an AMAZING start, really! You're going to have a much easier time with your kicks and jumps than other girls will, even if they have done cheer for a while (because although my kicks are good, my jumps are, uh, special :p).
Also, any amount of tumbling is GREAT!
In order to prepare, you can go on YouTube and search how to do some cheer jumps, and they do a great job of showing you how to do it! If you can get a head start in jumps, you are sitting pretty :p
Also, at tryouts, make your motions SHARP- don't make them ~flow~. Make sure to smile and act super excited to be there!
Consider lifting some small weights. because you could be lifting other people in the air for stunting.
Good luck, and I hope this helped. You're going to do great!! :D