I can't draw human faces look away Dx
Anyway, to the body: Don't use straight lines if you want it to look realistic, the human body has lots of curves. Also, the head isn't a perfect circle, it's more of an upside-down-egg-shape x3
Here's a little help about the proportions of the human body: http://en.wikipedia.org/wiki/Body_proportions
and there's lots of tutorials about it if you look it up. Also, try using your own body as a reference, too. Compare the sizes of your body parts, see how they move, see their shape, ect. It's helpful.
I did it the way we learned it in my school, should be about right XD