Download and install high Sierra

Boot your Mac in Recovery mode by restarting and when you hear the startup chime press and hold one of the following keyboard combinations until you see the Apple logo and progress bar.

How to reinstall macOS - Apple Support

• ❝ If you used Option-Command-R to start up from Internet Recovery, you might get the latest macOS that is compatible with your Mac. 

• If you used Shift-Option-Command-R to start up from Internet Recovery, you might get the macOS that came with your Mac, or the closest version still available. ❞

When the macOS Utilities window appears, select Disk Utility.

In Disk Utility’s menu bar click View > Show All Devices, then select the top-most drive device in the sidebar on the left (not an indented volume).

Click Erase and then select these parameters: 

• Use GUID partition map scheme.
• Name the drive/volume Macintosh HD,
• Use APFS format (for High Sierra, Mojave, Catalina & newer macOS) or Mac OS Extended (Journaled) format for Sierra and earlier OS X versions. 
• Click erase to complete the process.
• Quit ** and select Reinstall macOS.
• Follow the prompts to select the install destination drive and reinstall the OS.

Another way to reinstall macOS is to create a bootable USB installer for the OS you want to use. If your own mac cannot boot properly then you'll need to use another Mac that can run High Sierra. That would be most Macs released between 2009-2017.

Create a bootable installer for macOS - Apple Support

If you are installing High Sierra through Internet Recovery Mode, then you will need to use Fix#3 in the following article if you get a "recovery server could not be contacted" error message:

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

What is the exact model of this Mac? You can get this information by using the Apple check coverage page here:

View Coverage - AppleCare & Warranty