hi @ll !! i have created my first human head (not very beatiful ;-) )
make sure u don't have faces on teh inside of the head after u mirror and combine the 2 pieces, you have to merge the edges
also you need to use the split tool and split some more around the chin, head and side of face to take away the square ness. But its comming on mate...remember: faces and these kind of objects are the hardest to model so much patience needed