Basics
Shading can be a bit tricky at first but its not as difficult as it seems. The picture to the right shows a light bulb and several geometrical shapes; circle, triangle, cylinder, and a square.
You'll notice that one side of the shape is light and the other is darkened. All you need to know or keep in mind is just that. What the light doesn't hit, it is darkened. In this example, all you have to do is picture the light source and the object which is being hit by the light -- which part will the light hit? How far will the light go?
You also have to ask yourself where is the light source coming from? If the light is far above, the shorter the shadow is (try checking out your shadow at noon - 12:00PM) whereas the lower the light, the longer the shadow will become. According to the light source, make your shadow fit accordingly.
Good thing to remember also: what is the shape of the object I'm giving a shadow?Each of the shapes in the picture each have their own unique cast. The triangle has a pointy shadow, the circle has a circular shadow, the cylinder has a rectangular shadow, and the cube has a "L"-like shadow. At a different angle, though, the cube will cast a different shadow shape. For instance, if the light was head-on to one of the flat sides, it will cast a square to rectangular shadow depending on the light source's height.
With that in mind, you also need to remember: what is the shape of the object the shadow is falling on top of? The current example only has a flat surface on which the shadows fall but in most cases, shadows of - say a character - will fall on rocks or on water, which will look different compared to each other.