Also, don’t be afraid to use underlays. I know when I was a student, along with a lot others, I perceived that as cheating. But in reality it really helps you to understand how to build complex forms much faster and helps you to get an “eye” for correct proportions and perspective. After some practice you can ditch the underlay and sketch from experience. But I would definitely recommend pages of lines, ellipses, and cubes to start out initially–those will help build line control and confidence within your sketches.