-The most important thing doesn't have to do with any skill at all. A great way to show that you want to make the cheer team is to have a good attitude. Always be interested in what the coach, leader, or captains have to say to you. Volunteer for demonstrations & stuff. Answer your coach's questions & things like that. You basically just have to show that you're committed to the team.
-When you try out, it's important that you are dressed appropriately from your head to your toes. You should wear a sport's bra with either a cami top, tank top, or a semi-tight fitting t-shirt over it. You should wear either spandex shorts or soffee shorts or shorts that aren't too baggy. You should wear white, ankle cut socks. Coaches don't like it when you 'dare to be different' and wear rainbow knee-high socks. Your shoes make a lot of difference as well. You need to wear sneakers with a lot of traction & support. Don't wear Converse or Vans, athletic sneakers! Make sure your hair is up in a ponytail and out of your face with clips, headbands, bobby pins, etc. Make sure all of your jewelry is off, even the piercings your coach can't see, like a belly piercing. Those can easily get ripped out & it hurts a ton. Don't overdo it on your makeup. Foundation & mascara is really all you need. Cheer teams don't want the prettiest girls & they certainly don't want girls with makeup dripping down their faces.
-Before you try out, stretch a lot. I find it easiest to stretch when I'm watching TV, doing my homework, or listening to music. You will be really sore the next day since you probably aren't used to it, but it's important that you continue to stretch through that pain. That is what is going to make you flexible. If the pain is unbearable, take an Advil & put ice on the achy spots. Never ever use heat on your sore muscles. That makes you even less flexible than when you started out.
-You should learn the basics of being on a cheer team. They have names for certain arm angles and such. Some of the ones that my team uses is: L, touchdown, daggers, punch, box, half T, T, diagonals, high V, and low V. You should also be able to keep a beat and have good rhythm. You will be dancing a lot and it's important to look the same as the other girls. If you think you'll be a flyer, you might want to know what a liberty, arabesque, and a pop cradle is. You can look those up on Youtube & Google if you need visuals for those. The more advanced stunt moves are scorpion, heel stretch, bow & arrow, chin chin, etc. But you probably won't need those yet. As for your jumps, the most popular ones are a toe touch, herkie, and hurdler. They would rather see you with proper form than with high jumps. So focus on good form. You could learn that off of Youtube as well. I doubt you'll need to know much tumbling skill, but you should try to learn a cartwheel, roundoff, and a handstand. That's the most basic stuff you'll need to know as far as skills.
-Keep a good attitude, never give up, and good luck! :)