If you are new to RISC OS you should read Introduction to RISC OS.
The download link provides a copy of a beta RISC OS ROM image for the Pandaboard and Pandaboard ES. To get started with one of these boards, you need the OMAP4 firmware as well. This is how to get started – a full explanation of what to do to create an SD card that will let you start up with RISC OS on the Pandaboard.