Chisels are useful for mortising and doing some more precise wood working. The key to sharpening a chisel properly is to first understand the proper blade geometry. A chisel has a bevel and a flat. There are different kinds of bevels(hollow, flat, convex, and multi-beveled), but most will find the maintaining a proper flat bevel will serve them very well.
Here is one of the best examples. "Carpentry sharp" is good.