The distros used most often for servers (in no particular order) are: CentOS
, Ubuntu Server
, and Slackware
. All excel as a server OS, but have their own particulars, mainly in the areas of package management, location of configuration files, and availability of GUI tools.
Ubuntu is probably the most user-friendly for new users and also has more GUI packages available.
If you're going to use linux for a server it would be well worth your time to learn some command line, as this is where linux really shines. A few links to start:
The last one is a good read for windoze users thinking of migrating to linux.
EDIT: My personal favorite is Debian. They have a lot of good tutorials
related to server setups.