This tutorial will help you get started with drawing realistic human heads.
Most guides to drawing the head skip over the most important part: getting the overaall structure of the head correct.
To get the structure correct, you need to have a solid understanding of the proportions of the head and spacing of the features. To do this, you need to start drawing the head with its simplest forms. Getting features right is often a matter of making sure they line up on horizontal axes.
Copy the simple forms and diagrams below. Learn to draw the whole shape accurately.
Pay attention to the lines that cross the surface of the head. Make sure these line up. Practicing these basic forms will give you a base of understanding of the whole of the head. As you get more comfortable, add more detail and shading, but make sure the basic forms are in place first.