You can't go by the appearance of the image on the screen unless the monitor gamma is calibrated. This is a very common mistake unfortunately. Here's why:-
Basically you can fiddle with your monitor's brightness and contrast, and tweak the image's brightness and contrast until you have an image that looks superb to your eye, but what you see on the screen has no relation to what you send to your printer, as the monitor settings you make are completely arbitrary and unknown to the printer. Tweaking the image to compensate for the appearance on the monitor simply destroys the exposure values the image should be sending to the printer.
Bear in mind that unless there is a fault in your camera, the images will be correctly exposed, unless there's been a metering error. If they appear dark on your monitor, then that's because the monitor's brightness and contrast have been set so that correctly exposed images appear dark, not because the image are dark.
You must calibrate the gamma of your monitor by setting the brightness and contrtast to a known reference before tweaking images. At its most basic, this can mean setting the monitor's brightness and contrast (NOT the image's) until the display appears similar to the print, but it can be done a bit more scientifically with gamma calibration software.
Try this link
http://quickgamma.de/indexen.html for some free software to help with the process, and provide a more detailed explanation than can be offered in here.