Using the flash
The art of using the flash is more complex than one might think. Some people assume that you should only use the flash when you are indoors or in a dark setting. In reality, you can use the flash outdoors and in different lighting to get some pretty neat effects. Of course, you will be limited if you only have the on-camera flash to use, but nevertheless, you can still make a big difference in your pictures if you learn how to master the flash.
A great tutorial on how to use the flash can be found at photonotes.org.
Using a flash in any dark setting will make a subject more clear because there is more illuminated for the sensor to capture in the frame. Unless you have a very wide aperture, it is difficult to capture sharp and detailed pictures in low-light. It is also difficult to capture moving objects without using a flash unless your lens is very fast (wide aperture) and focuses extremely quickly. You can always set your camera on AI Servo to try to capture motion in low light, but odds are you will get a blurry photo. Flash can make up for what your shaky hands cannot.
A lot of people recommend being in P mode when you use the flash. This mode adjusts all the settings for you to ensure that your subject is illuminated properly in the foreground, however in a dark setting it usually means that the background won't be lit. The great thing about the P mode is that it allows you to hand-hold the camera while ensuring that the subject will remain in focus. If you want to get both the subject in the foreground and the background lit, you can try either bouncing the flash off of the ceiling or walls if you are using an external flash or you can set your camera to Av or Tv. These modes require you to use a tripod or other stable surface otherwise the long shutterspeed settings will make the results pretty out of focus.