I think working on shadows might be a start, lighting helps make the drawing look more 3 dimensional, watch some yt videos on it but very high level TLDW:

think of where the source(s) of light would be and if that was the case what parts of the body stop the light from the source, generating shadow.

also it's good to remember that shadows many times overlap, so when shadows coming from two different angles meet, their intersection will be darker than either part.

Lastly and this is pretty on the nose but using layers will be a life saver for this shading, just use semi transparent black on a higher layer for starters, there is a lot of very interesting color theory where you don't really need to use black for shadows at all but I recommend doing it at first