To do more with your 3pi than explore the demo program or use it as a serial slave to a master device, you will need to program it, which requires a computer and an external AVR ISP programmer like our USB AVR programmer.
Please see the Pololu AVR Programming Quick Start Guide for tutorials on how to get started programming your 3pi in Windows, Linux, and Mac OS X. That guide also covers the Pololu AVR C/C++ Library and the Pololu USB AVR Programmer.
Warning: Do not attempt to program your 3pi if its batteries are drained or uncharged (make sure you charge any new rechargeable batteries fully before you first use them). Losing power during programming could permanently disable your 3pi.