The Raspberry PI is supposed to be an educational tool.  For that, the good folks over as the Raspberry Foundation made a few linux installs available off the shelf.  You can always buy the pre build SD card, or if you are more adventurous can always cut your own.

I did this using the Debian Squeeze linux, simply download the image from http://www.raspberrypi.org/downloads and use Win32DiskImager to put the image onto the SD card.

Once running, you’ll get to the login prompt.

The username is pi and the password is raspberry.

At this point you’ll be at a command prompt.  not very helpful for a beginner, but don’t worry, type  startx press enter and you’ll get the GUI interface.