Ditto what Chelle said...open up the inside, trim out dead/dying branches, branches growing back inwards toward the tree, branches rubbing against each other (or one of them), etc.
If your tree is 15 years old and is only 12 ft tall I wonder if you have a dwarf, which would be grafted. Look for the graft and be sure to cut any branches below the graft.
Hope you get some good apples this year, sundina!
Shoe (whose dwarf apple trees grew 25 ft tall...boy did I get took, eh?)
