Figuring out how to correctly "model" objects with light has been one of the most difficult things for me. The thing that helped most was to learn the differences between how light interacts with matte and specular surfaces.
When painting a surface with both properties it sometimes even helps to deal with the properties of each separately, maybe even using layers.
Matte surfaces reflect light of their own local color (red objects reflect red light) and absorb all other frequencies. How much light they reflect depends on the surfaces' angle with respect to the light. If you treat a smooth surface like it is faceted, the faces that are angled most toward the light are brightest, and become darker until they are parallel with the light source. Any faces angled AWAY from the light source receive NO DIRECT LIGHT from that light source. The brightness of these faces will be the same regardless of the viewer's position. Here's an (ugly) example:
Specular surfaces, on the other hand, act just like mirrors and will reflect all colors of light regardless of the surface's local color. In fact, a specular highlight is really just a low-level reflection of the light source. Because of this, figuring out a specular highlight is sort of like playing a game of reflected-light-billiards. The highlight appears on the surface in the location where the tangent of that surface (or imagine faceting the surface again) is at the correct angle to bounce the reflection right at the viewer. The location of the highlight changes depending on the viewer's position relative to the object and the light. I've seen people make the mistake of placing the specular highlight in the same position as the matte highlight, but this is not correct! Hopefully the diagram below is more clear than my painful explanation:
And finally, to illustrate how the two kinds of lighting can work together on a surface---the left sphere has a matte surface only, with the light source directly overhead and to the left. The right sphere has the specular lighting added. Again, note that the matte highlight is on upper-left tip of the ball (can a ball have a tip?), but the specular highlight is down a little ways from it, where the surface is angled correctly to reflect the light source.
Sorry that was a lot of reading. I'll post a real painting soon to make up for it.