Good knives need sharpening less often than cheap ones, but don't wait until they are blunt!
Different people favor different implements and methods. There are many patent knife sharpeners available, but a traditional steel or carborundum stone is just as easy to use once you know how.
A steel often comes with a carving knife and fork as part of a set. Hold the steel rigid in your left hand. Lay the knife diagonally across it so the blade edge touches the steel at a 45° angle. Using a light, stroking movement, draw the knife towards you from the base to the tip, first on one side above the steel and then on the other below it. (For safety's sake always point the blade away from your left hand.)
The same stroking motion is employed using a carborundum stone (obtainable quite cheaply from kitchen or catering suppliers), the only difference being that the stone must be dampened before use.
Each 'tooth' must be sharpened individually, and this makes sharpening serrated, scalloped and granton edge knives a slow job. You can do it yourself, providing you own a sharpener which has two crossed steels (usually only electric or hand table models have these). Place a 'tooth' at the point where the steels cross and move it around slowly to sharpen each edge. This method of sharpening will wear down the 'teeth' eventually, so it is best to have the job done by a professional - he will be able to sharpen the 'teeth' and keep them distinct.