I'll have to assume you are trying to install Windows 8 and want to boot from a USB device to begin the installation (or fix a problem). Not actually run Windows from the USB drive.
You then say you have a Asus mobo but want to change your BIOS to boot from USB? Again I have to assume you aren't aware that there is no specific setting like that in your BIOS settings since you can only change the boot order. But you can select a boot menu! And to do that just press the F8 key (instead of the DEL key or something) shortly after you power up. Then select the device you want to boot from, assuming it's plugged in, and away you go. But that also assumes your USB device is bootable (and visible)! Otherwise, it will just cycle on to the next thing that might boot.
That said, I suspect what's really going on is that you haven't properly made a bootable Windows 8 installation on your USB device. And to make it I would suggest trying Yumi ( http://www.pendrivelinux.com/yumi-multiboot-usb-creator/ ) or even Sardu ( http://www.sarducd.it/ ) to do it. I hope I don't also have to point out that you will need a disk image in the form of an .iso file - but you probably already have that.
Hope it helps...