You need a bootable DVD or USB with a version of OSX or OS installer. If you don't have one that came with the computer (they stopped doing this a while ago) then you might have to buy High Sierra from the Apple store and create a bootable USB using another Mac.
You need a bootable DVD or USB with a version of OSX or OS installer. If you don't have one that came with the computer (they stopped doing this a while ago) then you might have to buy High Sierra from the Apple store and create a bootable USB using another Mac.