Is your montor properly calibrated? Have you made prints of your pictures?
Do you know how to use it on manual?
The problem here is that the camera is taking pictures automatically and will make decisions based on what it meters or what it has been designed to do. You have control on "automatic". Try using the programmed mode with exposure compensation set at +1 or +2 and turn on the flash. This will make it brighter. If you must ask how this is done, you really need to look at the manual that came with the camera. You will find most of these features in there.