Sketching simple geometric objects is a good way to get used to observing how light reveals form.
The main idea is to group together the areas in shadow and those in light, which helps if you have a direct light source. We humans are great at building a 3d form in our head with little information, and painters capitalise on that to let the viewer "participate" in recreating the image and also worry less about detail.
For this it's best to reduce the palette to two colours or even just one, plus white and maybe add more if you feel the solid aspect of the figure won't suffer. Here I've used burnt umber (brick red!) Payne's grey, titanium white and then a little yellow ochre at the end. Because it was a small painting I was able to make nice chunky brushstrokes for the faces of the brick.