Pruning is a science al by itself, and while many people realize that, some just go about cutting things how they see fit. And unless they are trained, they will see their fruit disappear or their trees die.
The rule of thumb is generally wait until late winter.
If you really prune a fruit tree while it is in season you will regret it, unless your goal is to do it in. The reason is simple, all the sugars are high in the plant. By cutting the tree now you would essentially be getting rid of all of that food used during the winter. And with the sugar goes the well being of the plant.
By waiting until winter you will allow the sugar to flow back to the roots.
It will recover from pruning much better if you realize that in winter, the roots are growing underground like the top does in summer, because all that energy in those top leaves are what it eats all winter, and is what you are taking away so it will starve in the winter, and may even die.
If you haven't taken care of a tree before I would recommend contacting somebody with experince for the initial work, then you can educate yourself and keep up with it in the future.
The Rule of Thumb Suckers
If you want to start with light pruning you can remove the suckers along the trunk. They are generally removed because they provide little added value for energy collectiong while taking too many nutrients away from your fruit.
When you are pruning a fruit tree, keep your outside branches, think of pruning it into a bowl. By properly directing future growth with your cuts will influence the shape of the tree for years.
And if you mess things up you may actually never get the chance to see it recover.