A good place to start would be with references from Pinterest or Google (though you should be aware of the copyright attached to images). I'd recommend breaking down the structure of the human body into basic geometry - abstract shapes, when put together, can communicate different forms including that of a human. Something to do if you can't find the pose you want is to pose yourself and take a photo. Animators in the industry often film themselves doing different actions for similar reasons, although pertaining to animation and realistic movements.
Anatomical knowledge is very important as a foundation for your work; I'd definitely recommend learning in your free time about the range of motion of joints in the body and proportions common among humans. Once you have this foundation, you can start to adjust proportions and exaggerate to develop your own style that reads well and looks extra satisfying.
Your sketches could use more variations in line weight, this will make them look less flat and introduce some contrast. You have defined silhouettes, which is great! I would recommend doing loose sketches of different compositions to develop more sense of flow.
Critiquing your own art is definitely good to get into the habit of, but I'd recommend restricting this to after you've completed a drawing so that you don't end up abandoning it prematurely.
Hope this advice helped!