So how do you shade? It's a good question, since sometimes I find myself making mistakes and it looks better with the mistake than the planned outcome. So what can I say other than experiment? Well fortunately there are a couple things you can do to get started in the world of shading.

The first thing to do, learn how to shade simple objects. I think a very classic exercise (and still useful) is shading a sphere, cylinder, and cube. Most objects around us can be broken down to one of those shapes (a sphere is kinda like a special case cylinder, but it's so frequent, might as well learn it than have to derive it from a cylinder).

Below is an example of a method of shading for those objects. no blending between values was used so you can really see how the shading is broken down. As you start out shading, I find it very helpful to draw an arrow indicating where the light is coming from. In big scenes this helps keep lighting consistent.

Highlights:
Highlights tend to be at areas "closest" to the light source. In math terms, we'd say highlights occur at areas where the normal of a surface points inbetween the viewer and the light source. Since we aren't computer programs, I take a fast approach and spots that are more flat to the light source get the highlight.

Core shadows:
This little detail can really push out an object and also determine the material of it. The greater contrast between the core shadow and the dark shade of the object, the more metalic/shiny something can look. The core shadow is caused by light reflecting off the ground, or a near by surface, and lighting up the edge of the object. Keep in mind that if there are no surfaces by the object, the core shadow may not exist.

As an excersize, you should try to make renderings like the example, and see how adjusting the highlight and core shadow can effect the type of material. Also, for technique, try making the shading smooth across the values (I'll try to post up an example later ;) )