I suggest you to focus on training to make the correct proportion. Your drawing is manga, but you still need the 'correct' proportion, am I right?
When you basically figure out how to make the correct one, then you can start train how to shade.
However, if you insist to train how to shade, I suggest you to pick simple objects such as ball, tea cup, softdrink can, etc and train how to shade those objects.
Shading is a long training. You have to train yourself to know where the light(s) come from, the tone of the light, and the value of the light. Then you gotta calculate (with your mind) where the shadow drop point, the shadow density, and color. Remember, shadow isn't black. Shadow is darkened tone of an object.
P.S: I've been drawing since I was small and still training on shading. Currently I do more digital drawing.