I consider myself a beginner digital artist, I am still learning the ropes but I believe there are a few very simple tips I've picked up on that can help other aspiring artists like myself.
Shading your sketch: When you draw your first sketch make sure that you show the areas that you want to be shaded, this will provide a guide to where you want to put your final shading
Contour Lines: After you have completed your concept sketch lower the opacity of that layer and make a new layer above that one. On this new layer draw the contour lines neatly and cover fine details such as eyes and clothing.
Shading: When I shade I make a new layer below the contour line layer but above the original sketch. Using the shaded areas in your sketch as a guide paint in the shaded areas.
This video goes over all of these tips, sometimes the best way to learn is to see it happen before your eyes!