How could you compute for the density of the human body?

You would need to make generalizations for each portion of the body, then find each volume. For example, call each leg and each arm a cylinder, your head a sphere, and your body a rectangular cube. Find the volume for each part using average heights and/or diameters. Then, divide the mass (kg or lb/(32.2 ft/s^2)) of the whole body by the volume you found. Voila, you have a rough density.
If you know calculus, you can be a little more accurate by doing a triple integral where the radius changes from the radius of your ankle to the radius of your thigh. Another way to look at it is if you dunk yourself under water and can compute the volume you displace, then weigh yourself, you can calculate your density D=m/v