Shading is all about creating shape, not making something darker, basically choose a light source and the closest shape to it should be highlighted where it is taking light, you should just look around and see how light reflects and stuff, for the face I would make most of one side dark with the top being dark because the hair is blocking the light. Then I would have some light on the opposite side of the dark on the face because the light is reflecting onto it, and possibly some where the features stick out.. the hair is the same, and with the phat it's similar but do it to each of the points. If you would like me to basic shade it to show you what I mean that is possible! Cool linework so far.