If you have trouble sharpening your knives at the correct angle you can give yourself an easy guide by drawing the correct angles on a piece of cardboard, mounting the drawing near your work surface and using a torch or other light source to make sure you're at the proper angle.
Instructables user Organikmechanic originally created this method to help sharpen honing chisels and planes, but the basic method works great for knives as well. Use a metal straight-edge held against the blade to see your blade angle superimposed by shadow on the cardboard drawing — this eliminates the guesswork that many have when first learning to sharpen knives.
The excellent Lansky sharpening system is another way to take the guesswork out of figuring out the correct angle, but it's a commercial solution.
Sharpening knives is one of those tasks that superficially seems easy, but involves a bit of geometry to get it right. Different blade edges have different types of grinds which require slightly different angles to sharpen properly, otherwise you can take off too much metal and possibly ruin the grind, requiring even more metal removal to fix.
Flashlight method for sharpening knives. [Instructables]