Download and Install the SteamOS Beta
Valve has set up a page with instructions on how to build your own Steam Machine. There are two download links there that give you two different options to install the SteamOS beta. Be sure to read the instructions carefully before choosing a download option.
SteamOS Beta is the first public release of our Linux-based operating system. The base system draws from Debian 7, code named Debian Wheezy. Our work builds on top of the solid Debian core and optimizes it for a living room experience. Most of all, it is an open Linux platform that leaves you in full control. You can take charge of your system and install new software or content that you want.More at SteamOS
I'm very glad that Valve decided to build on Debian 7, it was an excellent choice. The hardware requirements seem reasonable, and it's good that there are a couple of different install options.
Install SteamOS in VirtualBox
If you prefer you can opt to install the SteamOS beta in a virtual machine via VirtualBox. This will give you a chance to check it out without having to an actual install on your computer. The article below provides step by step instructions on how to install the SteamOS beta in VirtualBox.
Its my basic guide to help beginner users to install SteamOS on Virtual Box till finish from my experiences. If you follow this guide with careful you can figure it with yourself till finish.
My reason install SteamOS to VirtualBox: SteamOS is still in beta stage and purpose for IT experts only and I don't want to waste my time to install steamOS on my machine before Valve release SteamOS final version.More at Steam Community
You should be aware that you won't get the same kind of performance running SteamOS in VirtualBox that you would get running it on real hardware. But VirtualBox can be very useful if you just want to poke around and see what SteamOS is like without trying to do some serious gaming.