There is a quick fix when you feel your photos are too dark. The +/- exp on the top of your camera lets you add or subtract light from the photo. This button is called the Exposure Compensation Button and is the fastest way to adjust the exposure of your image. Most cameras can do this in small stops of .33 at a time. Hold the button down while turning the wheel at your thumb (or forefinger depending on your camera model) and dial towards plus (+) to add light to the image if it is too dark and towards minus (-) to subtract light if the image is over-exposed.

Look at the first photo below. It was shot normal, no exposure compensation. Since the subject on the table was backlit by the window, the image is slightly dark. By adding a bit of light with the +/- button, I can brighten up the image.

The photo below shows a real difference at +1.33. I added more than I really normally would have for demonstration purposes so that you can see the difference.