You may not be able to control your light source, but you can control where you and your subject are relative to it. Try to have your light source to the side of your subject. If the light is behind your subject, you won't get to see any detail -- instead, you'll just see a silhouette. On the other hand, if your subject is looking into the light, he or she may be squinting. Plus, direct light on an object or someone's face may be harsh and unflattering.
Photos with the best lighting tend to have the light source to the side. You'll want to make sure that they light source isn't too harsh -- otherwise you'll get shadows on one side of your subject. If possible, go for two light sources, one on either side of your subject. That way, the lighting will be even and you'll be able to see the subject clearly.
No matter where your light source is, before you snap a picture, take a second to look for any stray shadows. If you're shooting indoors and using a flash, move your subject away from any walls -- you don't want a shadow outlining him or her, even if the subjects themselves are well exposed.