You could start with three values for each different type of surface. Four makes it complicated. You could start with two values for practice.
Start with a simple overhead light source and think about what the light is hitting and what the light isnt. This is very very quick paint over.
And for art with figures in it you can apply the figure as an overall light object on a dark background, or an overall dark object on a light background.
Look at some more digital art and keep it up. 
