Download:
http://repo.steampowered.com/download/
What do you need to run SteamOS? Here's the hardware list from the FAQ:
Intel or AMD 64-bit capable processor
4GB or more memory
500GB or larger disk
NVIDIA graphics card (AMD and Intel graphics support coming soon)
UEFI boot support
USB port for installation
Note: one of the installation methods that Valve details requires a 1TB storage device.