How can I improve my faces/heads?

Sorry there's only one picture, I haven't drawn people in a while and this is the only recent image I have. I just want to know what's wrong with this and how I can fix it, and improve things like my hair/shading/structure/etc if possible.

Sorry there's only one picture, I haven't drawn people in a while and this is the only recent image I have. I just want to know what's wrong with this and how I can fix it, and improve things like my hair/shading/structure/etc if possible.