Why doesn’t an “on camera” flash light the background?
1. Since it is attached to the camera, the strength of the flash will only go as far as the subject
2. Backgrounds will be black because the flash only measures lamp-to-subject distance
3. Even automatic flash units will only measure lamp-to-subject distance.
So, as your photographic expertise grows, you will want to do away with most on-camera flash situations in favor of using off-camera flash setups.
Most camera manufacturers make flash units that can be used off-camera. They can be connected to the camera with a special wire or wirelessly. There are specialized brackets which take the flash off the lens axis and elevate it to approximate a “Hollywood” type light. This eliminates red-eye and gives the subject a more overall light. It is still harsh in nature, but at least it is coming from a different direction. The thing to strive for is to have flash units that are attached to light stands that you can move around to create different types of lightings and moods. Work with ONE LIGHT AT A TIME until you see what the effects of that light are. Then you can expand your lighting arsenal to more lights.
With all this said, you can also use your on-camera flash in bounce mode together with some kind of diffusion. This will soften the light and make the space look more normally lit instead of with that typical glaring flash look. Bouncing your flash off the ceiling or a nearby wall uses the ceiling's entire surface as a light source. That's why it appears softer. Bouncing your on-camera flash though, reduces its power. So, to achieve a properly lit room, you will have to increase the exposure by opening up the lens, slowing down the shutter speed (not always recommended where people are in the shot), or increasing the ISO number or a combination of the three.
Here are some examples of bounced light I used at a recent event:
Since there was very little light in this room, both were shot with relatively high ISO’s. The ceiling was a light color (not actual white) and the flash was placed on the camera but aimed up at the ceiling and tilted forward just a bit. I do this so that a little of the light will be directed at the subject, but most of it is bounced. Note the soft quality of both shots and, of course, no shadows!
Keeping all this in mind, for better images, start thinking about taking your speedlights off the camera.