What should I do if I want to be a cheerdancer?

I want to be a cheerdancer at my school's Sportsfest next school year (2010). I can move, but I'm still pretty stiff. I can't high kick, cartwheel, do Russians or split, but I want to learn how. What can I do to increase my flexibility (so I can do the things above), get really rhythmic and build up my confidence so I can make it in next year? What extra-curriculars should I join? Right now I'm thinking kickboxing and hip hop, but I'm not sure. HELP!