I always recommend Proko's tutorials on human anatomy (and just his tutorials in general) when you're still a beginner. They're free on youtube, and can give you a wealth of knowledge and experience.
There's a lack of understanding here in the functionality of anatomy, motion of the body, weight, and how fabric folds. Pretty sure he has videos for all of that stuff. You can also check out Mike Mattesi's 'Drawing with Force' books/videos/etc. I have had the pleasure of his oversight personally, and he is an excellent educator with a ton of knowledge that will help with your rigidity in pose.
Start simple, learn how the body is put together and how it moves, then go from there.