There are currently two versions of SteamOS available and Valve recommend that you use a default installation method which is a pre-configured image-based install using CloneZilla. You will need an Intel or AMD 64-bit capable processor, 4GB or more RAM, 400GB or larger hard drive and an Nvidia graphics card. Support for AMD and Intel graphics is coming soon but it is not available right now. Valve lists UEFI boot support and USB port for installation as additional features.
The other method uses Debian Installer, which allows for customization after an automated install step. Some Linux knowledge is highly desirable for the second option. The default installation file is a 2.4GB ZIP file that needs to be installed from a 4GB of larger USB stick formatted in Fat 32 name named SYSRESTORE. You will have to unzip the SYSRESTORE.zip file to the stick and boot from it.
Once you boot you will be asked to select Restore Entire Disk, but the disadvantage of this method is that it needs a 1TB disk for itself. Both installation methods will erase all content on the target computer which means you need a free 1TB hard drive to give it a try.
Even if you install, we believe it won't be the same to play games via a keyboard and mouse and at this time there is no way to simply buy a Steam OS controller. We might give it a try later this week, we just need to wipe the content of one of the drives that sit in one of the old drives in the drawer.
You can get some more detailed step by step guidance at the official website here.