You're off to a good start with the shading. The primary thing to keep in mind is to pick a direction that the light will be coming from and stay focused on that. I haven't tried it myself, but I've heard that it helps for beginners to draw an arrow on the page in the direction of the light. That will help to picture where the light is falling on the characters and where it's not, i.e. where to shade. Keep in mind not only the shape of the head in general, but where things interrupt the light, such as hair nose.

I hope that helps. You have a nice beginning here, and some more shading would really help bring it to life.