Wow, for your first digital picture, this is very clean!
As you continue to practice and do more digital pieces, perhaps try to add more than two tones to your shading and constantly reference your light source (the shadow on the nose in this piece is from an opposing light source). Also, try building the underlying shapes of the head and facial structure with the shapes you use for shading. Doing this will give a more realistic structure to your drawing.
In any case, I'm very impressed. Your line work and colors are very crisp and clean, especially for your first digital piece. Keep up the good work!