The first step is determining whether your knife's edge is a V-bevel or convex. (See the illustration.)
The edge on most knives is a V-bevel (we're including the chisel grind in this category, since it's half of a V) and will require flat sharpening stones or rods. If your knife has a convex-ground blade, you'll be choosing a stropping system, including sharpening compounds.
Next, you'll need to choose the level of "abrasiveness" of the stones, rods or compound you'll use. If you're starting with a truly dull edge, you'll want to begin with something relatively aggressive. A simple touch-up job, on the other hand, could be done with something finer.
You'll also need to consider the type of steel your blade is made of. Many of the current crop of "super" stainless steels -- M390 and ZDP-189, to name two -- are notoriously stubborn and require more abrasion, especially if the edge is deadly dull. Ordinary carbon steels and some tool steels, like 1095 and A2, are friendlier and yield more easily to medium- and fine-grit stones and compounds.
A word of caution: If you're new to knife sharpening, you're likely to choose something more aggressive than you actually need. Until you develop a sense for your knife and your sharpening system, we advise starting with a finer stone or compound. If that doesn't work you can always move up to a tougher abrasive, but you can't put back steel you've enthusiastically ground away. Take it easy.
Last but not least, a few knife patterns require specialized sharpening tools and techniques. The American tanto (with its multiple bevels), serrated edges (which can't be honed with flat stones) and recurve blades (S-shaped edges) are a few examples. If you have such a knife, be sure that the sharpening system you choose can accommodate it.
To learn more, visit our Knife Sharpening page.