first steps

The general structure I think is a great starting point. It looks to me like you are trying to focus on structure and where the important joint landmarks are—my recommendation would be to try to stray away from so many straight lines between the joints themselves. C-curves to help give a flow to the human figure do wonders for reducing stiffness. I highly recommend looking up some of the videos on gesture and anatomy posted on YouTube by Michael Hampton, as he has some amazing guidelines for reducing stiffness while still implying the form. 
Form, then structure, and then detail. Big broad simple strokes reduced down to more finished art.

This is a great start! Length and proportions are looking good!
To get a bit more fluidity in your work try your best not to use any rigid straight lines especially for the legs and arms, try instead to do the full length of the leg or arm in one curved stroke, adding the joint points in after. I noticed too when you go to further the drawing by adding more form to the arms and legs you're sort of drawing them in one equal width log, instead of that try to make the form stop and start again at the joint points. For instance an arm, draw the upper arm first getting a bit thinner at the joint points really paying attention to the form of the arm from the reference then start to the the lower part of the arm keeping them as their own forms.
I hope that makes sense!