The basics: Figure construction is the theory that if you learn to draw and render basic shapes in perspective like these:
That you can then take those basic shapes and combine them together to make the basic structure of any material form you can see or imagine, including the human figure!....
This applies to the human head and it's anatomical components:
This construction principle also applies to the entire human body.* The key is to have your shapes in perspective and to have those shapes which represent parts of the human body in the correct proportion to each-other.
*The other half of drawing which gives a piece of art life is rhythm and gesture. Eventually you'll need to master that as well, but in the beginning getting a good handle on being able to construct material things from basic forms is the place to start.
Proportion:7-8 heads is the classic "realistic" ideal.