drew struzan traces. keep that in mind when you see killer illustrations.
now im not saying every artist out there traces but a lot use reference as well. also drawing circles and straight lines are meant just to warm you up.
what i would do (and used to do myself) is draw your hands and feet. theyre both pretty tricky at first but eventually you'll get the hang of it, and it should come naturally after a few pages that you should then be able to draw a few from memory!
once you've got that down extend it to the rest of the human form. Use reference obviously.
also check other artists works and see if you can borrow some of their techniques/style and incorporate it into your own.
best of luck!