Select your hard drive from the left column in Disk Utility.

Click the "Erase" button at the top of the window.

Choose a name for your drive (you can rename it later). Leave the scheme as GUID Partition Map.

If you are installing an SSD, choose the APFS format. If you are installing a mechanical drive or if your device doesn't officially support the newer OS, leave the format as Mac OS Extended (journaled)

Click "Erase." When the process completes, click "Done."

Exit the Disk Utility by clicking on the red X button on the top left corner of the window.