Katie,
You could be great...just work as hard as you can and do your best. You don't have to start out in shape because ballroom will get you into shape and keep you in shape. It will be hard and sometimes your muscles will be sore but it is a good sore.
The great part of your age is that you'll learn the choreography much more quickly than most of the rest of us and your body is still growing so it can shape itself to be a dancers body!
Good luck and have fun dancing!