Editor’s note: Part Two of the Debian Guide is now online.
An Illustrated Debian 7 Installation Guide
First of all you need an installation image (ISO file) which you can download from www.debian.org. I recommend the small network install image. After downloading the file, copy it to a blank CD, DVD or USB memory-stick. For information on creating a bootable USB memory stick, see the article Create a Bootable Live USB Thumb Drive or Rescue Drive Using UNetbootin.