Why are my pictures dark?

So I developed 8 x 10 pictures and they end up being dark, but my screen is much brighter? Can someone please explain why? Is it because I should calibrate my monitor to match the picture and how do I do that? Do I just lower my brightness level… (nikon d90)

You say "developed", which means film, but then you mention a digital camera?
I'm guessing you actually meant "print", instead.

Who printed them? Did YOU do it at home?

There are too many unknowns here to answer properly. Start by uploading a couple of the photos that ended up too dark, and give us a link. Also give clearer information so we can help you better.

Your screen image is brighter because their is light coming though the image instead of light hitting the front of the image like the print. You would need your monitor to be calibrated to get a better sense of what the colors will look like. To do it professionally you would need to buy kit. But a "cheaper" way could be to just look at the image on the screen and make it even lighter to compensate for it being dark when printed. Are you printing on your own or through a store? They can always pull up the image at a store and change the brightness if you bring it in and talk with them before they print.

I believe these articles will help you:

http://www.shutterbug.net/techniques/pro_techniques/0211bwprint/index.html

http://www.shutterbug.net/techniques/digital_darkroom/0209monochrome/index.html

http://www.shutterbug.net/equipmentreviews/software_computers/1010datacolor/index.html

http://www.imaging-resource.com/ARTS/MONCAL/CALIBRATE.HTM

http://www.drycreekphoto.com/Learn/monitor_calibration_tools.htm

Your monitor must be calibrated if you expect to make good prints. In my opinion a subscription to Shutterbug would be a good investment.