One of my favorite classes to teach is how to use the Elements of Design.  The are the building blocks that make up your painting.  The ones that I like to work with are: Value, Color, Shape, Line, Texture, and Pattern.  In each class we focus on one of the elements and emphasis that element.
Value is a good one the start with.  Value is simply the lightness or darkness of colors.  Using value pens we make small value sketches of our subject.  Here is an example of a value sketch.  Notice that it is just a simple sketch using white, grays and black.  This shows me where I want to emphasis the values in my painting.
Next we paint our painting using one color and just focus on getting lights and darks with that one color.

Here are some photos from our fun value class.