There's lots of info out there on tree pruning some of it is good, while other information is just plain wrong. It isn't that hard though. And it basically comes down to this; you want to cut away any branches that are growing in toward the center of the tree.
The reason that these branches are removed along with any branches that will grow to rub against each other is that they reduce the tree's overall efectiveness.
When you prune fruit trees like this you are giving them a chance to invest their engery into fruit production. You can really thin fruit trees and they will recover just fine though you may miss out on some fruit production.
Other trees however, for example, plum trees have to be carefully pruned because little "branches" stick out where the fruit grows on the next year. Different fruit tree's need different kinds of cuts to better maximize fruit production, even the space you have it in can determine what cut to use.
If you have never pruned before, or you have never pruned a specific type of tree make sure that you do a little research for the types of trees you have. I would suggest that you read Cass Turnbull, he has an entire book on pruning, and delineates how different species have different tollerance to pruning.
Be warned, though. Once you know what a badly pruned tree looks like you will realize that most of them have suffered some really terrible pruning.
It has the potential to make you sort of sad.