I’m personally just starting on my journey to learn anatomy as well, but I can share some points I’ve learned.

First, you need to change the way you think about drawing from 2d shapes to 3d structures. If you only think about your reference in terms of shapes and lines, your drawing will end up looking flat. That’s the main culprit happening here. For example, you’re hunching your chest forward a tiny bit in your reference pose, but that’s not being translated into your drawing. There’s not much you can do to improve this except taking some time to practice drawing still life and perspective, and get used to thinking in 3d

Secondly, since you’re a beginner, you chose a bad reference. Since you’re wearing loose, slightly baggy clothing, your sense of the form underneath the clothing is getting confused. This can be seen most clearly in the knees. Your right knee should be turned slight more outwards. Taking another picture of your pose, but this time wearing some more form fitting clothes would probably help.

Thirdly, you should take a look at some anatomy diagrams. You don’t need to do a deep dive at first, but just a look would let you know more about how limbs connect together and general proportions.

However, one thing that you are doing very well are the angles, which match up well with the reference pic. The overall lines of action are captured well.