I would let it grow the way it wants the first year, then cut it off about one meter tall early next spring. When it sprouts out again, keep an eye on it, and thin out the new shoots until you have 3-4 strong branches evenly spaced around the trunk, and no less than 15 cm apart. That will make a strong, low-branching scaffold that will make it easier to tend and pick later. Pruning and training the 3rd and 4th years is designed to prevent crossing branches, Y-crotches, watersprouts, and other such problems. Seedling nectarines usually take about 5-7 years to bear. Remember that the nectarines you buy in the store are named varieties: they are grafted from the best seedling of thousands at the breeder's farm. Sometimes their offspring inherit their good qualities--and sometimes they don't!