You will need atleast 8GB of USB storage, and you have to make the USB" Boot-able" it includes writing some commands in CMD etc.
If you have C: and 1 or more partition(s) then move all valuable stuff to drives, leaving C: with just Windows files, as it will be formatted.
The rest, i'll Google an easy step-by step link, just gotta prepare for Sehri
EDIT: Here is the link, using this method, you won't have to use CMD for commands:)
http://pcsupport.about.com/od/windows-8/a/install-windows-8-usb.htm
All you gotta do is download W8, torrent etc.