Well, it actually depends on which kind of illustration and style You're aiming to. Any artist, reference etc. thereabout?
This drawing seems a neat, simple, interesting cartoon / comics kind of style not of the manga type, but doesn't say much about your end target. On this specific drawing I may say that the face outline is probably too thick and that if You may vary apprpriately thickness by hand pressure on the tablet You should make lines thicker where You would expect the shape to turn away from light source rays, thus suggesting, synthetically, a shadow effect.
To be clearer, if light is supposed to come form upper right corner, make line thicker on the opposite side and more where the tangent is normal to the light direction.
Obviously You may find any kind of free tutorial online and select what You're looking for, as a style or subject.
Anyway, welcome and keep going!
Panta rei (everything flows)!