Download the image:
First step would of course be that to choose and install a linux distribution on the SD card.
I'm personally a RedHat/Fedora user since quite some time but I thought, for the RasPi, to give Debian a go. I decided to install Raspbian “wheezy”, an optimised version of Debian for the Raspberry Pi, and it comes with LXDE as default desktop environment, for those who care about it.
You can download the image from the RaspberryPi Downloads page.