Start with darkness. Take out all the default lights and remove the global ambience. If you are working outside, this is obviously much more complicated, because you cannot just remove the sun. That's a whole other discussion to focus on.
Add your key light. The key light is the main illumination on the subject. It should light the focal point of the scene whether that's an object or an individual. The key light will define the most visible lighting and shadows. It's your dominant light source and often takes the place of the sun, light from a window or ceiling light. Offset the key light 15 to 45 degrees to the side of the camera. Make sure the subject will not look directly into the key light.
Add the fill light. The fill light will help soften and extend the illumination of the key light. It will help make the subject or the rest of the scene more visible. The fill light can simulate light from the sky or secondary light sources (such as lamps). The fill light generally is set from the opposite angle to the key. The fill can be raised but never above the key light.
Add the back light. The back light, like the name applies, will sit behind the main subject. This light will help reduce contrast and help make the object more 3-D looking. Usually the back light sits lower than the subject.