I culled this from a web site. If you google for 'prune apple tree' you'll get enough info to keep you out of mischief for quite some time.
Keep the simple reasons for pruning in mind when you head outside with the pruning shears, and you'll do just fine. Your aims are to create a strong tree that will bear the weight of all those bushels of apples, to allow sunlight to penetrate the tree so that all your apples have nice red cheeks (unless they're 'Yellow Delicious', of course!), and to keep the branches low enough to reach without hauling out the ladder. Most young fruit trees are already trained to a sturdy basic shape when you buy them, with main branches selected and in place.
All you have to do is thin the growth as the tree fills out, and occasionally snip out the top of the tree or taller branches to keep it down to size.