Start up on recovery mode (command +R).
Click on Disk Utility. (It may be under Utility Menu on the menu bar)
Erase the disk.
Exit disk Utility
Install New OS from Internet
A simple internet search will lead you to...
Is it time to sell or give away your old Mac? Or do you just want a fresh start to clean up your machine? Here’s how to securely delete all of your files, then install a fresh version of macOS.
www.howtogeek.com