Looks like you're doing pretty well, and you maintained proportions for the most part. I would work on the general 'shapes' presented by the human figure. Particularly how the muscles curve. The biceps and calves could use more defining, as well as the joints in the knees and elbows! I would recommend practising your anatomy, but I really think that's all you need: practise. The more often you draw stuff like this, the more confident you'll become, and that'll help with how 'shaky' some of your line work looks.

Good luck with practising artwork, and have fun ^w^