Unlike apples and pears, you are not trying to create a central leader (dominant vertical branch). Peaches and nectarines are trained to an open, spreading, vase-like shape. See the illustration below for a before and after look at the branches of a young peach or nectarine tree.
Take out at least half of the new, lateral shoots to do this. The remainder of those shoots will produce your fruit for this year.
If your trees set fruit this first year, pick off some of the immature fruits, spacing them about 8" apart on the branches. This will allow the fruit to reach optimum size and improve vegetative vigor. Fruit thinning in the future is also important for the very same reasons. Less is more. If you don't thin, you will get many more fruits than the tree can handle, resulting in broken branches and small fruits. So don't be afraid to thin. The resulting fruits will be fuller and much nicer.
In later years, you should continue to "shape" your tree. Peach and nectarine trees are best trained to a spreading, open, vase-like shape. This is the natural way your peach tree will want to grow. Peaches and nectarines are unique fruits in that they bear on one year old wood. So it is essential that you prune them hard to allow for adequate sunlight penetration into the tree. This is turn invigorates the tree to grow more young wood within the tree that will produce next year's crop. Those branches that grow this year are the ones that will set flower buds in the fall. Look at the diagram for the Mature Tree Form. That is what you are aiming for over the first several years.
The best time to prune peaches and nectarines is in the early spring. Try pruning after the last frost date for your area. At this time, most of the winter damage can be trimmed off and you will minimize the effect of late frost damage to your buds and blooms.