Since apple trees are very long-lived, it's important to establish a strong scaffold in the early years. Pruning also depends on your individual requirements – whether you desire a low branching tree with apples you can pick easily from the ground or whether you want a full-sized landscape specimen.  Wandering deer may also require a higher branching tree. Remember that apples develop small spurs which produce the fruit and will fruit for several years, so be sure to retain some each year.

See a step-by-step guide at About.com