For help creating a bootable external drive, have a look at Apple's page How to create a bootable installer for macOS Just be aware that if you want to properly erase the hard drive that is also your startup drive, you will have to start your Mac from an external media first.

Click the Format pop-up menu, then choose a file system format Click the Scheme pop-up menu, then choose GUID Partition Map. In the sidebar, select the storage device you want to erase, then click the Erase button. Choose a file system to which you want to format your external drive

Select the disk or volume in the list on the left panel. Click on the Erase tab (next to First Aid). Home Erase external hard drive Mac Mac File Eraser - Powerful Mac OS Cleane