I need to reinstall High Sierra?

Does your mac have a working operating system.

If it does then you can download a copy of the full installer for macOS High Sierra.

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on macOS High Sierra.

you will be redirected to the High Sierra download page of the App Store.

It should say Get or Download.

(If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.)

This will download the Install macOS High Sierra.app to your Applications folder.

Make a bootable USB flash drive installer using the Install macOS High Sierra.app

in the Applications folder and the createinstallmedia command in Terminal,

How to create a bootable installer for macOS – Apple Support (UK)

Having created the bootable USB installer, make a full back up of your mac.

Restart your mac while pressing and holding down the option/ alt key.

In about two minutes you will see the Boot Manager, select the USB and

press Return. The mac will no boot to the USB.

From the Utilities panel select Disk Utility and press Continue.

Click View in the menubar and select Show All Devices.

Highlight the Disk (Apple HDD..... or similar) not any of the indented Volumes.

Click Erase.

Give the Disk a name.

Format: APFS.

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Click on Install OS and press Continue.

The installation procedure should start follow the prompts.