Portrait photography or portraiture in photography is a photograph of a person or group of people that captures the personality of the subject by using effective lighting, backdrops, and poses.
As with all photos, light is the key. Then comes composition and focus. Get your subject in good light first, then start taking photos. If you are outdoors, golden hour (hour before sunset or an hour after sunrise) is the best time. If you are indoor, look for light streaming through a window or use lights.
Some helpful videos: