Johnnie is right, key light is usually about 5 o'clock or 8 o clock, and the fill is about half the intensity and on the opposite of the key. Since most webcams will probably have an "auto iris" or auto exposure, you can brighten yourself up by being in front of a medium to dark background. If you stand in front of a white wall it will average the exposure for the entire screen and you will be dark.
I run into sits like this all the time videotaping Judges wearing their robes. many are older white haired, or follically impaired men, couple that with a black robe and its an exposure nightmare.
You would have better results using your video camera, taping it, and then editing the footage in something as simple as windows movie maker. That way you could also use a better mic. though with consumer camcorders these days none but Canon's ZR800 have a mic input, and even then you have AGC (automatic gain control).
that's why for my home use I usually use my Sony VX3 3CCD Hi-8, and converted to DV on the way into the PC.