I know the struggle to start with digital art. I'm drawing with digital watercolor mostly and it took me some artworks to get used to it.
I don't know the program you're drawing with (sorry ;_;) but I may help with some tips.
I can see parts of a rough sketch shining through. When I'm starting with a new piece, I'm usually sketching roughly and redo my sketch on a new layer and set it to multiply (layer settings - hope your program has such things. If not, do your coloring on a layer beneth it.)
Next, I'll do my base colors and very roughly sketch shadows in. For me it's easier to start with shadows and adding lights close to the end. For most of the time in digital art, you're doing "blending" - means picking a color between the two colors you want to mix.
Also, shadows are either warm or cold and opposite to the light: warm light = cold shadows and vice versa. Means, light and shadows are not on the same tone-range as the base tone. Light is usually (not always) desaturated and the shadows are saturated. These tips helped me making much more realistic pieces c:
But you're on a good way - shadows and light seem to be logically correct c: Really like the design of the head! x3