Hi, there's this guide on Sparkfun:
http://www.sparkfun.com/tutorials/330
I haven't read it but I assume you sample the voltage generated from the piezos (when hitting them). Normally you would apply voltage to a piezo but they actually work in the opposite direction too.
You can so sampling using the ADC (Analgoue to Didital Converter). If you need help on how to use the ADC, Dave recently made this excellent post with a simple example:
http://forums.netdui...tal/#entry45571
Off-topic: Most stuff work somewhat in their opposite direction:
* Certain types of electical motors become generators
* LEDs can detect light (a small voltage is generated as light hits the diode)