I've been programming in Processing for a few weeks now. I started out by looking at the examples in the learning section. For example, if you're interested in animation you might look at:

http://processing.org/learning/examples/linear.html

http://processing.org/learning/examples/bounce.html

http://processing.org/learning/examples/nonlinear.html

Or maybe you want to see how to interact with Processing:

http://processing.org/learning/examples/mousefunctions.html

Take a a look at those. Run them. Modify them. There are tons of examples available from the Processing IDE through the File->Sketchbook->Examples menu. While you're modifying the examples, keep the reference manual handy, which is available through the Help->Reference menu. The manual is very good about giving working code snippets for each function.

The other option would be to patiently wait until Ben Fry and Casey Reas finish writing their book on Processing, which I think is slated for some time next year.

Processing may look intimidating at first glance, but it really is quite simple and elegant. Just jump in to it and start experimenting. In the short time I've been working with Processing I was able to write a 3D program that interacted with the user through a web-cam and provided feedback through a particle system and billboarded text. Right now I'm finishing up an interactive Sudoku solver. I've been learning just by looking at the examples and the reference manual.

If you get stuck on something ask specific questions. The community is friendly and supportive.

Good luck,
-Chris