Specify the boot device in the host system BIOS
If your host system is not already configured to boot from a CD, DVD or USB drive, then you must configure it to do so before you can boot from LiveCD or LiveUSB, as appropriate.
The next time you restart the system it will boot from the device that you chose, so long as you insert the bootable disk first. Thus you can boot from a LiveCD or LiveUSB disk image.