Learn to Draw Digital Sketchbook is a pretty interesting app that teaches you how to draw in step-by-step lessons. You select from the provided drawing tools, follow the spoken and text instructions, and you're on your way.
You can also dive into the app and draw freehand to apply your skills. You can draw on different types of "paper" and vary the size, pressure and hardness of the drawing tools. Drawings can be saved to your photo library or emailed to friends. This is the kind of creative use of the iPad that I really commend, and it's nice to see a clever and highly educational approach to learning. One note: the app only runs in portrait mode. There may be some good reasons for that, but since I keep my iPad in a case that is oriented to landscape, I would have liked the option.
I don't know much about drawing, but I was able to turn out some passable sketches after going through the lessons. The app is from Walter Foster, a company that has been publishing art instruction books for decades. Best of all, the app will get you started for free, but if you want to move much further, you need to get some in-app purchases of additional lessons that cost anywhere from US$2.99 to $6.99. The quality of the lessons is high, and the app is handsome and well organized. About the closest I get to art is landscape photography, but for someone just getting started in sketching, this app is worth a look. You can check the gallery for some screen shots.