The easiest and most productive shape for an apple tree is maintaining a central leader with 3 sets of "scaffold branches" roughly 3' apart vertically. A scaffold is a set of 3-5 branches radiating out from the trunk in all directions (within a vertical span of 1-2 feet). This allows sufficient light to filter through for optimal apple production and air circulation.
You do not want to prune more than 30% of the tree in one season, so the priority for removing branches is:
-Dead or diseased branches as soon as you notice them; sterilize pruners (10% bleach) between every cut when removing branches infected with fireblight
-Suckers growing from the base of the tree
-Structural pruning: crossing branches cause the most problems, and then any Y-shaped branches. Once those are all removed, move on to branches that are straight up or down (ideal branch angle is 30-60*)
-Thinning: remove branches that are growing too close to each other - imagine how the light travels through the tree. Start at the top of the tree, since light needs to filter through to get to the lower branches, and work your way down.
Always prune at an angle upwards towards a healthy bud that's facing a direction you want the branch to grow (cut just above the bud), or cut just above the collar of a healthy branch intersection (only cut the smoother wood; the collar is the swollen interlocking wood grain that encircles the base of a branch).