Summarize:

If you already know what angle your knife is sharpened at, you'll probably wish to sharpen it at this angle again. Sharpening at a different angle will take significantly more time and may take a few goes before any rough angles are smoothed out. If you don't know the current angle, ask the manufacturer of your knife or inquire at a knowledgeable knife shop to determine what angle is appropriate for your knife. If you have to make a gut decision, choose an angle of 10 - 30 per side. Shallower angles make a sharper edge that doesn't last as long; steeper angles are more durable, so 17 - 20 is a good compromise between the two.
Pick an angle to sharpen your knife.