A common mistake is placing eyes too high. In a standard head, the eyes sit halfway between the top of the skull and the chin.
This determines where the "highlights" go and where the "core shadows" fall.
If you are looking to start your journey into portraiture, "Drawing Portraits for the Absolute Beginner" (part of the popular "For the Absolute Beginner" series by Mark and Mary Willenbrink) is widely considered one of the best entry points. It breaks down the intimidating task of drawing faces into manageable, logical steps.