Be confident in your abilities & ready to learn. Also ALWAYS have a positive attitude! You don't want to look nervous or doubt yourself, but you also don't want to walk in like you know it all and are better than everyone else. If you are focused on learning the material, polite to others and follow directions/ are aware of what you are supposed to be doing at that time then you will be fine :)
It looks bad to the coaches though when girls are complaining all the time, don't do stuff full out when they are supposed to be going over the material or are too busy talking to friends rather than listening to directions. It looks bad to the judges if you are too quiet/nervous (staring at the floor rather than the judges/whispering the cheer/messing up 50 times and saying "sorry can i try again" rather than just finishing it through the best you can after 1 retry), or give them scary crazy facials like some allstar kids do( just smile big and don't go overly crazy with facials: no winks, fish faces, looks like your eyes are going to pop out of your head, etc.)
As far as appearance, think All-American Cheerleader.... that means hair pulled back in a clean high pony with a plain ribbon (no bangs or whispies hanging in your face), soffee shorts, sports bra and T-shirt. Natural makeup, socks, cheer shoes and NO jewelry. They will tell you what the specific tryout attire and colors are i'm sure :)
Good Luck and let us know how it goes!