The Canon dedicated external flashes are very easy to use and give much better results than the built-in flash because you can easily get them to bounce light off the ceiling or an umbrella while retaining automatic control of the flash. Regardless, when shooting flash you should use manual ("M") mode for best control of lighting. For example, let's say you're shooting a small group indoors. Set the shutter speed to the max (typically, that's 1/250 sec) so that you will minimize motion blur. Then adjust the aperture to be small enough that everyone's face is in focus (this will vary, but f/8 is a good guess for a small group in a small room). Finally, adjust the ISO up and down to increase and decrease the contribution of ambient lighting to the shot. If ISO is very high, then the flash's contribution will be relatively small. If ISO is very low, then the background might turn out very dark or even black. Before shooting like this, I make a few test shots. If there is too much noise at a high ISO setting, you might want to dial it back and increase exposure with the shutter speed (1/125 or 1/60), but I find motion blur a bigger problem than a bit of noise, so I wouldn't go slower than that most of the time.