study anatomy, study how the muscles work, study how the skeleton works, draw LOTS from real life. Draw people. Try getting into a couple of nude figure drawing classes. that will help a LOT.
if you are looking for a reference book. "Human anatomy for the artist" is a good book. The Loomis books are also indeed good. Also Burn Hogarth's "Dynamic Anatomy" as well as "Dynamic Figure drawing" will help. Just don't get too into his stuff, because his students are notorious for drawing exactly like him; you don't want that, you want to be able to put your own opinion in your art. Hogarth however is in my opinion one of the most masterful artists there is for rendering the body, the way it works, and the way it moves. The best way to make yourself better at drawing people though is to draw them a lot. You learn from your mistakes, and the more you make the same mistake, the more clear it will be what you need to do to fix it. (it takes a long time to master the figure).