If you are new to drawing don't do the more complicated things so fast. Start out by drawing 3-dimensional shapes like a sphere, cylinder, cone,pyramid. Shade them appropriately and cast shadows. Look up reflecting light and several other things.
Then slowly build your way up to being able to draw individual bones. After you draw individual bones start drawing them together in various poses. Remember how bones work how they dont bend or break. Then when your done drawing your skeletons draw the muscles on top of those bones and draw how they contract and move to create motion and what they do.
Then when your finished with all that you can start drawing the flesh on top of everything and these portrait drawings. But remember how the muscles move and how the shape of the bones look because even though you cant see it you need to be able to make account for everything underneath that skin.
Drawing is a lifetime learning process you cant skip things and learn it fast.