Welcome to Gaia! ::

There is no way to make yourself taller, except standing taller. But if you are short and also skinny, you'd be perfect for being a flier, the girls who go up in stunts.
For flexibility, there are plenty of videos and blogs online about exercise routines which help with flexibility.

Here is a key to cheerleading that a lot of people don't realize until they do it themselves: You must have good muscle mass. Not only are you running around, doing dance moves, and lifting or being lifted into stunts, but the entire time every voluntary muscle in your body must stay as flexed as you can get them. You will be soar at first, but evetually you will get to the point where it actually feels good to flex your muscles, or "stay tight" as we put it.

For being loud, a good exercise it to lay down on the floor, preferably of a large room or gymnasium/auditorium, and speak as loudly and sharply as possible, and have someone tell you if they can understand what you are saying clearly. Do that until they can understand what you are saying.
You might want to look up some cheers online to practice with.
ALSO, please note that when you say a cheer, it is not like singing or speaking where it is okay to slur together a couple of words here and there. It must be like this:
GREEN. GOLD. AND. WHITE. FIGHT. FIGHT. FIGHT.

Also remember to be very respectful to your coach, coaching assistant, and the veteran cheerleaders.

I'm also considering do cheerleading too, but I have no clue how to do any gymnastics, I'm more of a dancer. I used to be a cheerleader back in 7th and 8th grade. I'm very loud and pretty flexible though and I'm short. It shouldn't matter if you're short (I'm only 5'2). What you could start doing is stretching everyday to become more flexible and practice basic cheer moves/stunts. As for being loud... I honestly have no clue, it's natural to me. sweatdrop

For the cheering portion, don't focus on making your voice higher pitched if that isn't how you normally talk. Lower sound frequencies travel farther and are easier to hear and understand. Also, don't yell from your gut. The proper way to do it is with your diaphragm (Located just below your lungs). Place a hand there when you're practicing cheers on your own so you learn what it feels like. 3nodding

I did a little bit of cheerleading in middle school and tried out for the high school squad, but didn't make it (I couldn't do gymnastics either and I wasn't extremely comfortable stunting; I was more into the cheers and dances). However, I was in a sorority in college. Sororities do house chants/cheers and have to be heard from blocks away, so you definitely learn how to do it correctly. xd