Agree with Sprucegum. In addition:
1) first take out anything that is dead or diseased.
2) take out anything that crosses and rubs another branch
3) take out anything off the bottom of a branch
4) shorten branch by cutting just beyond a growth nub that is pointing the direction you want the branch to go.
Have also heard that if you have old trees that need a lot of cutting back you shouldn't cut more then a third of the tree in any one year. But have seen apple trees cut back almost to stumps and come back just fine. Your mileage may vary.
