3-Point Lighting: A classic lighting setup for film and documentary.
The "key light" is the dominant light source in the 3-point lighting system. Position the key light first, either 45-degrees to the right or left of the camera and slightly angled down onto the subject.
Next, set up the "fill light" on the opposite side of the key light. The fill light's purpose is to soften some or all of the dramatic shadows produced by the key light.
Lastly, set up the "back light" behind the subject and off to one side. The back light helps to separate the subject from his or her background and can be used to accent the subject's hair.
For more detailed information, read this article.
(image credit: theonlysilentbob via Wikimedia commons)
NATURAL LIGHT: When you are in a hurry, natural light is your best friend. Shoot outside (in a shaded area so your subject isn't squinting) or shoot indoors next to a large window.
POSITION: Avoid positioning lights directly behind or in front of your subjects. Lights positioned 45 degrees off to one side of your subject and angled slightly down at your subject will produce the most dynamic footage. When shooting outside, avoid positoning subjects with their back facing the sun. You want the light to be falling on their face.
REFLECT: When you only have one light source, try reflecting the light back onto your subject to lighten up any shadows on their face. You can use a either professional reflector or a large piece of white foam core.
DIFFUSE: In general, diffused light is more flattering for your subjects than contrasty light. You can diffuse light by using a softbox, an umbrella, or even a large white sheet (definitely not recommended when using hot lights).