LiveUSB

First you need to download livecd iso image. Gentoo livecd or Gentoo based SystemRescueCd are good choises for installing Gentoo. SystemRescueCd supports both 32bit and 64bit processors on the same image. Almost any modern livecd should work.

Get the livecd image
You need to change the architecture to match your cpu (x86, amd64, arm, sparc).

Write the livecd image to the USB drive
Now you can boot. You must select the USB drive as startup device in bios.