Others have handled the direct answer, but I think a more useful answer is to use more light. Cameras record light. They are really not see-in-the-dark devices.
For portraits, you don't want to have the subject in the sun, its too harsh, but you can take photos on cloudy days, or in the shade and get good results.
You don't have to spend much money, but you should get some lights. Off-camera strobes are a good place to start. The strobist site has tons of information, tutorials, etc. http://strobist.blogspot.com/
I strongly recommend that you not just buy Nikon's latest and greatest strobe and put it on the top of your camera body. Its a better flash than the built in one, but you want it off camera.
There is a decision between manual and automated strobes/flashes, addressing that is a topic that can fill a book.