It's actually a pretty good example of how the very sophisticated evaluative metering system in your camera can be fooled by such a relatively simple and everyday situation. The basic problem is the camera is metering from several spots in the scene but most of them are in the darker areas, so it sets the exposure for those areas, ignoring the face which is your main subject. Good as it is, it can't think, but you can.
As suggested above, you could dial in a minus amount of exposure compensation - you may need to chimp a little to get it just right.
You could switch to spot metering and meter on the face which wont be far away although you may need to compensate slightly for the difference between caucasian or darker skin and 18% (mid) grey.
Another way is to remain in evaluative metering but move right into your subject so that the face fills the frame, press the exposure lock button - move back to your original position and take the shot . As the system has metered from several areas of the face it should be pretty accurate.
The elegant solution is to switch to manual mode, move into your subject and fill the frame with the subject's face. Now balance the exposure display with the cursor in the centre of the viewfinder scale. Now move back to your taking position ignoring the the fact that the metering display will be wildly out of balance, but it doesn't matter - you've metered for correct exposure on the face - not the whole scene. The advantage of this method is that as long as the light doesn't change you can now take as many shots as you like from this position of the same subject knowing that your metering is right for the skin tones.
