Read this tutorial to learn how to shade step by step realisticly starting with 4 basic shades.
This tutorial will teach you how to shade realistically. This technique can be used with almost any medium, digital and traditional. To start out, pick out four shades to work with. For this tutorial I have chosen skin tones because I will be drawing a face. You will need to do this for every color that is going to be in your painting or drawing.
First, make a rough sketch of what you're going to draw. When drawing this, outline the shadows to help with the next step. The color of the sketch doesn't really matter. I chose a dark sepia for mine. Sorry the lines are confusing, you'll be able to see what's going on a lot better in the next step.
Using the darker shadow color, I filled in the areas that I outlined in the previous step.
Next, fill in the rest of the areas with the midtone.
Then smooth out the bits in between the midtones and shadows. I made the hair and eyes peach as well because I'm lazy. :)
Now fill with same color used to smooth out the shading (shadow 1), paint reflected light in the shadows. It may take some practice to get this right.
Finally, add in the highlights in the appropriate areas. This is the final product. I hope you learned something!