I previously proposed a couple of drawing exercises: gesture and continuous line. An important third aspect of drawing (and painting): value and creating believable lights and darks!
It’s great fun to form shapes entirely with with lights and darks – placing darker areas behind light and vice-versa (forming the boundary rather than drawing it). Look at the form of the geometrical shapes at the bottom of the following image – there are no lines: only boundaries between light and dark. (With this exercise, I started with the white of the page and blocked in from white – I could equally have started from a mid tone.)
Here’s my reference: http://drawingacademy.com/wp-content/uploads/How-can-I-improve-my-rendering3.jpg
The image also includes a simple approach to forming a sphere – again light, mid tone and dark!
Some artists prefer to work on 5 values and this is fine too: http://www.robinurton.com/blog/wp-content/uploads/2014/04/sphere-value-diagram.jpg
I’d say 5 values is an absolute maximum to start with…the key is to keep your value scale as simple as possible. It’s easy to blend between value zones later – after the main image is formed.
Have a go at some geometrical shapes to see how easy it is to manipulate values!