Unless you have professional flash equipment and proper lighting, flash is your worst enemy
Use flash only if you are OK with the picture not really being "real" but for documenting events such as in parties.
The reason flash sucks is quite simple:
Human eyes perceive light is by following shadows
Flash throws light from the same direction as the shot is being taken thus throwing the shadows exactly behind the subjects hence the camera sees no shadows and hence the subject appears not real but a cardboad cutout
One use of flash is actually when it is very bright outside and the sun is throwing a horrible dark shadow on the face of the subject. Use flash to compensate!