As seen in the comments, it looks like your camera is unaware of the flash, at least in some operating modes. Research in your user's manuals for an answer. If not, maybe your camera/flashgun combo cannot use TTL (through the lens) flash regulation.
Another possibility is that some of the hotshoe contacts are not working (i.e., dirty) and that is preventing proper operation (provided that your flash is actually 100% compatible with your camera).
Let me briefly introduce a couple of useful concepts: Types of light regarding time duration.
Continuous Light: Is the one coming from a constant source, like the sun, an incandescent bulb, a stable, flame, etc. This kind of light can be considered constant, as its intensity will not vary a lot through the time that takes a "normal exposure".
Pulsating Light: For photographic purposes, is light coming from flashes and strobes. These are pretty obvious to explain, but consider that the light pulse duration is very close to the exposure time for a typical shot. This is very important because of the way different types of light interact with the light regulation methods your camera has:
Aperture: It regulates the amount of light that enters through the iris at any given time. For example, an aperture one stop "more closed" lets in half the light, no matter if the time considered is a second, a minute, or a day... as long as both measurements are taken with the same time. Remember that Aperture also controls the depth of field.
Shutter Speed: It controls the amount of light by controlling how long this light is let in. It means that a shutter speed "one stop faster" opens the shutter for half the time, thus letting in only half the light (assuming a perfectly fixed continuous source of light).
Sensitivity: This is the ISO setting. This does not affect the amount of light but how the recording medium reacts to a given amount of light, no matter if it's analog (film) or digital (sensor).
When you work in a rather luminous location but are also using flash, you are combining two types of light, continuous and pulsating, thus, you can control how much of every type of light reaches your sensor of film by choosing the right method: Aperture or Speed.
One of my photography teachers used to say: "You control continuous light with shutter, pulsating light with aperture".
Shutter speed affects "only" continuous light. Pulsating light is theoretically unaffected by shutter speed because the pulse has a fixed duration. This duration is extremely short, so adding more time to the exposure only lets in more continuous light, if there is any. However, there are practical limitations due to mechanical and electronic delays and response times. Normally, as long as you do not exceed your system's synch speed you'll be fine. (This speed usually hits between 1/125 - 1/180, but there are a lot of exceptions)
Aperture affects both continuous and pulsating light, and also controls depth of field. Sensitivity also affects both, but has a side effect in contrast quality and graininess of the final image.
When you shoot a scene, you have to define what's going to be your main dominating light, and set your camera accordingly. Usually it is "easier" in such difficult conditions to work in Manual mode, since you can control all three parameters: shutter speed, aperture, and sensitivity.
Aperture mode, most of the time, gives you a shutter speed that will catch enough ambient light to get a "properly exposed" picture. If that ambient light plus your flash's light is over exposing, it means your scene is likely illuminated enough without the flash. This is because the flash's pulse duration is likely fixed and very high, so, decreasing shutter speed (keeping it open more time) just lets more ambient (continuous) light in.
If you choose a more closed aperture (higher F number) the camera will let in less light, both ambient and flash, but also increases the exposure time, so after the flash pulse finishes, it will continue gathering ambient light. If the subject or the camera moves it will result in ghosting. If you choose a more open aperture (low F number) the camera will let in more light and reduce exposure time, likely resulting in over exposure)
If you choose shutter priority mode (S or Tv) and go to a high speed setting, the camera opens up the iris (lower the F number). As you increase the shutter speed, the less ambient light gets in your picture, and flash now dominates the scene, but if you still get too much ambient light, you will eventually hit your system's sync speed limit (the camera won't let you set a faster shutter speed).
Try going to manual mode, and use values approximately like these: ISO 100, f/4.0, shutter speed 1/80-1/125, and shoot without flash. If your scene results close to well exposed, then, that's the problem, there is too much ambient light. These values I give are my starting point when I try to shoot indoors at night (under typical home illumination) with flash at approximately 3 meters/10 feet.
If with these settings you still want to use the flash then:
- Increase the shutter speed so you get less ambient light in the picture, your test shots without flash will be clearly under exposed, but remember there is a limit given by the synch speed.
- Use a small aperture to reduce both lights (ambient and flash) until you're satisfied with the results.
Now, depending on your intended result, you may also try:
- Change your shooting time, so the sun is in a different angle, giving you less intense ambient light in your location.
- Change location for a less illuminated one.
- Re-orient the beam of your flash so it bounces from the ceiling or a wall instead of hitting directly on your subject.
- Use a diffuser in front of your flash so the light is softer, broader, and less intense.
- Place the flash further away from the subject (if it's on your camera's hotshoe then step back and zoom in, it also helps reducing perspective distortion and blurring the background).
EDIT: I did purposely let out that you can also regulate your flash's power. Usually your camera will let you set 1/1, 1/2, 1/4, 1/8, 1/16, 1/64 of its power (some cameras may not allow all the settings or may have more settings), either for the integrated flash or the external one. If the flash is external, you can also set it in manual mode and set the power level there.
