Hello and welcome :)
It's good that you started a sketchbook here in the forums, people will give you helpful feedback like that of Darktiste, I would suggest as well that you take a look at some helpful resources. Check out the resources section of the forum. Also in order to help you I would like to point out some words that you might find helpful and maybe you already know.
foreshortening: method of rendering a specific object or figure in a picture in depth. The artist records, in varying degrees, the distortion that is seen by the eye when an object or figure is viewed at a distance or at an unusual angle
I'm pointing out this not because I think you don't know it, just because I didn't knew it until very late on. Also you should check some tutorials on how to approach gestures. There are some pretty useful sites for that like :
QuickPoses.com and those that Darktiste pointed out earlier. Try checking out your proportions often. I also tend to screw up on that part. Based on what I'm seeing, your shading technique should be more black depending on the depth you want to give. Usually what we often mistaken is the fact that reality / nature has a great deal of value range, arranging black blacks and white whites all around us. Try not to stick only to middle grays and middle pencil variations. I tend to use B for outline, 2B-5B for refine lines and last 6B-8B for dark shadows.
That's all for now, I hope you keep on updating your sketchbook in order to see your progress. Till then keep up the good work and keep on practicing on daily basis.