go to this website and download the one that suites your computer
http://distrowatch.com/table.php?distribution=debian The latest release of Debian is Debian 6
If you have a 386 system then click on the i386 download that
If you have a 64 bit system then click on the x86_64
Now with Debian (Linux) there are several different types of desktops, as you will see, If you have256 megabytes of ram or less then download any of these,
Fluxbox, LXDE, Xfce, WMaker, Openbox,
If you have 512 megabytes or more then download either KDE or Gnome, Gnome is the easier one for new Linux users. Once you get used to Linux then you can download KDE from the install software, with Linux you can download all the different desktops and pick which one you want to boot up into once you boot up into Debian, you can even have Mac desktop by downloading the Docky Bar. Debian is good but there are better distributions available for new Linux users, like
Ultimate Edition 2.9 This is a brilliant distribution with mind blowing 3D Desktop graphics
Linux Mint Debian Edition,
Sabayon 5.5
you can get these from the website above