Try a few, or all, of these "values" exercises in your sketchbook. I've included examples from my own sketchbooks . . .
1) Draw some DRAPERY, or draped fabric -- Use charcoal, charcoal pencil, or a very soft drawing pencil (6B). Try to have at least four values in your drawing -- lightest (the white of your paper), a light medium gray, a darker medium gray, and a black. . .