I generally prune mine in winter-time - now is as good a time as any, but if you only had 20 or so apples last year, is it really big enough or old enough to need more than a tidy-up? When we bought our apple trees, the supplier told us that apart from a good handful of bonemeal in the planting hole to start with, and making sure that the area around the tree is kept free of other plants, he wouldn't recommend feeding, however, in March, when the buds are well on their way, I give mine a handful each of general fertiliser (Known here as vegetable and fruit fertiliser, and based on ammonium sulphate) sprinkled over the branch-spread area surrounding the tree, and forked lightly in.
If you don't have a problem with a soil that dries out, don't use a mulch, since (apparently) it provides cover for small mammals such as voles and mice, which can damage the tree at root level.