If you have decided that you would like to be a cheerleader, then you will need to attend cheer tryouts. Also you should make sure you have all materials required such as...
Proper clothing(whatever your coach prefers)
Tennis shoes and socks
Possibly cheer poms
Bottle of water to stay hydrated
The only other thing you will need to do is to show up on time, get stretched, and then you are ready to go!
After you have made the cheer team, your coach will assign a certain day for your cheer practice! At practice you will learn many cheers, chants, jumps, and stunts. Kind of like in the picture below. Then you will perform it in from of your school either at a pep rally or at a football game.