Is it a free-standing tree, or trained against a fence?
In either case they are pruned in early summer, and it MUST be dry when you do so to avoid the chance of disease spores entering the cut wood. Apart from removing dead, spindly or crossing branches, the basic principle will be to remove fruited wood and replace it with young growth - peaches fruit on year-old wood.
I'm sure someone else can help you more - fruit isn't my forte.