Cel shading is a flat looking 3D image, with a cartoonish look exemplified by animated movies.
The basic elements of a cel shaded effect are :
- Sharp shadows
- Little or no highlight
- Outline around objects
There are many types of cartoon effects, from the dark and gloomy streets of Frank Miller's 'Sin City' to the colorful scenes of Anime or Disney.
The first step is to decide on the mood. There is no better way to do that than doing some research... so start watching cartoon, read comics and decide which atmospheres suits your needs :)