It depends on what kind of camera and flash you are using, but for a DSLR with a Hotshoe flash- Adjust your shutter speed to the desired fastness, depending upon whether you need a fast or slow shutter speed. Next, adjust your aperture. Your aperture will determine how much light is being allowed into the camera, so if you are using a very bright, non-adjustable flash, you may want to use a smaller aperture. Some flashes use a TTL (Through The Lens) mode of metering, which should take out some of the guesswork. Some flashes also allow for turning the intensity up or down. Turning the intensity down will eliminate a blown-out photo or portrait. When possible, BOUNCE your flash to reduce the flatness of the image. Bouncing a flash off of a wall, ceiling, or a white piece of paper set at 45Degrees of your subject will greatly increase the softness of the light, eliminating harsh shadows, and giving everything a much more pleasant fullness.