Which distribution would you use for a server?

It depends largely on your personal experience on one hand and your need for professional support on the other hand. If you are good to go and get your hands dirty you probably want to look into Debian min, Ubuntu min, FreeBSD or the like. If you need professional support you want to check Collax, Red Hat or Novell and friends. I feel like the current "shooting star" Ubuntu gives you a nice and flexible way with their free system with the option for payed support.

If you already use a major Linux distro on a regular basis, I'd use the same distro for server. Since you would be already familiar with the particularities of the distro, your problems would generally be limited to server-specific questions, which would generally apply to any distro. There are 2 main groups of major distros : Those rpm package based -- redhat/fedora/centos, mandriva, suse, and Mageia (I include Mageia because although forked from Mandriva a year ago, it is very stable, and is developed by most of Mandriva's former development team.) There are also the deb package base distros, primarily debian and ubuntu. I would suggest using an rpm or deb package based distro, since it would be relatively easy to install any server-related package that is not already in the selected distro. For ease of support, as suggested above, I would favour ubuntu over debian. for rpm-based distros, Mageia over the others, due to disruptive changes being currently introduced at Mandriva, and much more friendly utilities in Mageia/Mandriva than the other rpm distros.

If you are newbie I think that ubuntu desktop and you can install lamp server, for the first step you can find a documentation here: https://help.ubuntu.com/community/ApacheMySQLPHP on ubuntu os in help browser you will also find ubuntu server gide! And if you want advanced os you can install debian it's very stable redhat or centos is hard for newbies lucky

Ubuntu Server LTS.