Start training fruit trees as soon as they are in the ground. Proper training keeps branches bathed in needed sunlight and builds strength to eventually hold the luscious bounty.
Train a fruit tree either to a central leader form or a vase form. The central leader form is like a Christmas tree, with a single main trunk off which grow successively smaller branches. This form suits apple, pear and sweet cherry. The vase form, in contrast, has three or four main limbs radiating upward and out atop the trunk. Peach, nectarines, apricots and most plums are inclined to this form.
If your new tree is just a single stem, right after planting cut that stem back to about 3 feet high. This stimulates new shoot growth and branching. Create a central leader form by letting the topmost new shoot become a vertical continuation of the main trunk. Some of the resulting lower shoots will become permanent side branches. For a vase form, three or four branches of about equal vigor will be the permanent ones, with the trunk ending just above the topmost of these branches.
In either case, select branches to save, then remove all others. First remove any branch originating lower than 18 inches from the ground. Branch height does not change with tree age, so you want the lowest branches high enough to permit mowing or mulching and to keep the fruiting branches off the ground. Also remove branches where overcrowded. Ideally, branches spiral and are well-spaced around the trunk. The ideal branch also makes a wide angle with the trunk, indicating a strong attachment. If a well-placed branch makes a narrow angle with the trunk, spread it carefully by inserting a toothpick or notched wooden plant stake between it and the trunk.
Select and prune branches as your tree grows in its first season. For the vase-shaped tree, those three or four branches you keep will become the main limbs of the mature tree. No other training will be needed. For the central leader tree, each year treat the leading shoot like a new tree: Shorten it, then select side branches just as you did the first year.
If a new tree already is branched, shorten the best-placed ones, then prune off others right after planting.
The goals in training a fruit tree are not only to give it strength and good shape, but also to get it bearing as soon as possible. Pruning can delay fruiting, so do no more than is necessary.