Jeans doest fit me??

i went to old navy! i am size 8 (12 year olds)! size 8 could not fit me! and i try 10 and its look too big on me! i have a problem finding the right jean! ARGGGGG! i want jeans are cover my shoes (half way) ! telll me where to get the right jeans! AND I HAVE NO IDEA WAT SIZE AM I!