John,

You should prune pretty much every year and ideally you should prune in late winter when the try is still dormant.

There are lots of instructional videos around explaining how. I like the Stark Bros. videos for a quick explanation if you don't know where to start googling.

The quick version is this....

Apples need a central leader type pruning so they look like a classic ball shaped bunch of branches above the trunk with a main strong branch in the center. (compared with pears and other trees that have a vase shape)

In general, on a young tree, you want to trim back 1/4 of the last years growth near a bud with a 45 degree cut.

In general, on a young tree, you also want to space the branches about 6 inches or so from each other to keep them spaced properly.

You might be a bit late in the year to do pruning, but like I said, you should make it an annual part of your tree care.