Ubuntu Server. Free, Doesn't use many resources and very very well documented.It is also more stable, lighter on resources and it is compatible with nearly all hardware out of the box.
However,these are some of the system requirements;
- 700 MHz processor (about Intel Celeron or better)
- 512 MiB RAM (system memory)
- 5 GB of hard-drive space (or USB stick, memory card or external drive
but see LiveCD for an alternative approach)
- VGA capable of 1024x768 screen resolution.
- Either a CD/DVD drive or a USB port for the installer media.
Or else you can try to figureout more on these as well for your future benefits:Ubuntu Server(mentioned already above), Debian and CentOS;are all great choices. Ubuntu has a lot more "stuff" in it, so if you are looking for something semi-lightweight for a VPS I would recommend Debian. It's package management is top notch and it will run fine.
CentOS is also a good operating system for explicit serving, if you are looking at running LAMP stack on your server, it is also my recommendation. Most other distributions will sink in quality a bit or be too difficult for a simple server even though they may offer maximum customization (Arch, Gentoo).