Start with a young tree. Formative pruning is done in WINTER. I will assume you are going for the bush (1-2 foot trunk) or half standard tree (3-4 foot trunk) on a semi dwarfing roostock, by far the most sensible and easily managed tree. Tall trees with a long trunk, say 5-6 feet, (big rootstock such as MM111 or 25) between the ground and the branches are called standards and are pretty and 'traditional' but hard to manage as you need to get up a ladder to pick and prune-such trees are better as garden features than fruit production units.
Plant the tree in winter, mulch and water if the first year is dry. Do not allow a newly planted tree to carry any apples in its first year, snip them off. You want a reasonably symmetrical framework of branches coming out, the lowest should be at least 18 inches from the ground. Branches with very upright angles are more likely to break, remove then in the favour of more horizontally inclined branches. Do not allow too many branches to develop, remove those which are too close to others. Look down on the trees from above to get a view of how the branches are distributed, try to achieve symmetry. In the second year, consolidate this work by taking out any badly placed branches, those which cross others, any which overcrowd the centre of the tree. Remember to aim to keep the centre of the tree open to light and air for optimal fruit development.
There is additional material available from the RHS in relation to winter pruning: