The first light to position in a three light setup is called the "Key."

The Key is the brightest light source and is positioned 45 degrees to the left or right of the subject.

Generally, the the Key Light is above the person's eye level, and lights one half of the person's face.

In the example at the right, notice how one side of the subject's face is dark and the other is lit.

The next step is to fill the dark area...