A very quick and dirty tutorial to give an idea of how shadows fall on faces. The subject is more complicated than this, but this will get you started in the right direction.
* The bits that stick out, like the nose, and the edges of the lips, and the eyebrows, and the cheekbones on some people, will tend to cast shadows onto other parts of the face.
* To get shadows right, you have to carry in your head an idea of how the face and head looks in three dimensions. You may find it easy to start by getting a doll with a molded head, like a human-shaped action figure or fashion doll, and a flashlight, and shine the light onto the doll's head from various angles. Or even by taking a flashlight into teh bathroom with you and shining it on your face and studying it in the mirror.
* The above shows you what it looks like with one source of light. If there are multiple sources of light, they will each cast their own shadows that interfere with each other. It's best to actually shine two or more lights on a person and see how the shadows lie unless you're really familiar with it.