Sketchbook Express for the iPad is a very solid art app for free. You have access to a variety of brushes, three layers for working, color selection, and more. There is the Sketchbook Pro version which costs $4.99 and gives you access to far more tools, but if you are just starting out or are working on iPads in classroom setting this is a very reasonable app for starting and teaching basic art concepts with digitial media.
Compatible with the iPad. Requires iOS 5.0 or later.