Drawing is all about proportions, shape, and practice, practice, practice!
There are several methods for dividing the human figure. The most common (but not necessarily the easiest) is the head division system. Here, you use the head as your point of reference. That is, once you've established a head size, the rest of the figure will follow that size.
The image above follows an eight head proportional system, commonly used in illustration and comic book drawing. A seven and a half head system is used in fine art situations, and a nine to ten head system is used in fashion illustration.
Another, very helpful system doesn't use the head as a reference point, but rather uses specific parts of the body as reference points. This system is as follows:
1. Create two marks about 2" from the top and 2" from the bottom of your paper. This locates the top of the head and the bottom of the feet.
2. Draw a line joining these two points. (This is your center of gravity.)
3. Divide this line in half. (This marks the bottom of the pelvic bone.)
4. Divide the bottom half, in half once more. (This marks the center of the patella, or knee caps.)
5. Divide the top half into thirds. (The top third marks the top of the head to the clavicle or collar bone; the middle third markd the clavical to the bottom of the rib cage; and the bottom third marks the bottom of the rib cage to the bottom of the pelvic bone.)