What can I wear for a Halloween party?

I'm 15 and am going for a Halloween house party soon, but I desperately need a costume! My friends are going as Dorothy (from the Wizard of Oz) and Cinderella and Pink Ladies, from Grease, etc, but I have no idea what to go as. I don't want to go too 'slutty', but not a complete freak! haha, please help!