My Boyfriends Pre-Debs Is After Crimbo And I Duno What To Wear Something Not To Dessy Or To Casual ?

My Boyfriend And His Friends Are Just Wearing jeans And Shirts But I Dont Know Any Ov Thier Girlfriends So I Dont Know What I Should Wear. I Dont Want To Be To Dressed Up In A Formal Dress But I Dont Want To Be To Casual In A Skirt And Top ? Help Pleasee