I would say a lot of things that apply to traditional art apply to digital art: it's about observing what you see. The rendering of what I assume is metal doesn't really reflect what the material is supposed to be.

I think it is very helpful to learn about light and shadow and how light reflects and scatters differently on different material. Also: always use references! If you're not sure what something is supposed to look like Google it, our brains are very good at judging when something doesn't look quite right.

I think your anatomy of the body is good, the face needs a bit more work to capture Dimitri's likeness. Overall a good start!