See this:
http://alexds1.deviantart.com/art/Face- ... -109889834
And this for details:
http://out-of-the-rain.deviantart.com/a ... l-61880083
Deviantart has alot of tutorials, just do a serch for what you need.
http://browse.deviantart.com/?qh=§i ... e+tutorial
I know from experience that you tend to place the the ears wrong until you get a feel for 3d-dimensionality. Not only in height, but it easy to place them too far front.
The basic rules for proportions are, eyes in the middle, lower end of nose in the middle between eyes and lower end of head, and mouth in the middle between those two. Upper part of ears is located just below the eyes.
Now, you probably want this help for 3d-modelling, but I would still suggest practicing by asking your parents or friends to model and sketch their heads. To get a feel for the shape. Do it with different time restrictions, one, two and five minutes. The limited time forces you to focus on the important details and hightens your understanding of the basic anatomy. It's important that you never let yourself continue after the set time has passed.
Good luck.