internet recovery

Just to add to Ronasara's fine advice...

 Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

Internet Recovery Mode is accessed by pressing the Command + Option + R keys on startup. Be aware that it will only install the original operating system that your Mac initially came with. If you have been using something newer, that original OS will then have to be upgraded.

Ah, good luck my friend! :)