Not knowing the brand of computer you have I'd simply say you have to enter the BIOS (sometimes called Setup) by tapping the F1, F10, Esc or Delete key during a restart. You move around in the BIOS by using the arrow keys below the Delete key, Enter key and pageup/pagedown keys. You have to disable 'power up/wakeup on LAN' or similar language. Exit saving changes.
Please reply back to this as to how it went