I've found that when starting out modeling a head, it's helpful to insert a background image and simply draw the head by extruding a vertex around it. Then you can add depth to the head and make it look realistic. These two videos got me started with the basics of modeling a head from scratch.
To add a reference image to model from, first press numpad 1 to go to front view. If the view isn't already orthographic, press numpad 5 to switch to orthographic. Then, press the N key to open the right toolbar, and scroll down to "Background Images". Check the box, then expand the menu and click Add Image. Set the Axis as "Front", then click Open to open the image you want to use. Do the same thing for the left, right, and back views, but make sure the image you use for those views is truly of that part of the head. For example, make sure from the left orthographic view you can see the image of the left side of the head. Once the image is added, you can change the opacity of it with the slide under the background image menu.
Hope this helps!