you need to define a light source, that is where the light will be coming form. I think you've pillow shaded a lot of the trunk and branches which means the light is coming from you, try doing it from a corner or but a moon in the background and use that as a light source
the overall structure of the tree doesn't really look like a tree, the mirrored branches dont do it any justice. Unless your going for a style of your own, you might want to consider a shape more like the one in this photo:
Clickyhave u tried to get a reference? It may be a good idea to do so, google image search is always good to try
edit: link works, thanks Ryan