Characters have a defined way of writing the strokes, so you look them up by number of strokes and radical. ("Radical" being, usually, the bit on the left or the top)
You can see a web-based example at [2]. Click on the "Radicals" button in the upper right. You'll see a list of radicals (sorted by stroke order), and clicking/unclicking on them will cause the list of kanji below to change.