Basically I sticks to these 4 steps when it comes to pruning my fruit trees (all of which are 3 years old);
1) Remove any branch that exceeds what I can reach on my step stool. The whole point of the fruit trees is have fruit that I can reach easily.
2) Remove branches that are growing vertically. The more horizontal the branch, the more fruit it will produce.
3) Remove broken or downward facing branches. These produce little if any fruit.
4) Remove small, crossing branches (really just twigs) from the middle of the tree. These produce lots of leaves but little fruit and restrict sun and air flow into the middle of the tree, which an lead to disease in the summer time.
After pruning my fruit trees, I spray them with dormant oil to kill overwintering insects and fungi (which we have a lot of here in San Diego). Since I am almost out of space, here is a link on using dormant oil;
3 year old peach tree before pruning
Peach tree after pruning
Photos by Doug Kalal