You can either just download the latest version of macOS High Sierra, or, if you've downloaded older versions before, find previous versions of macOS, which would be listed and available for download under the 'Purchased' tab in the Mac App Store. Next, you need to download a macOS installer from the Mac App Store.
Once you have your USB drive, you need to wipe it, which can be done with just a few steps. It is also better to use a USB 3.0 flash drive, as both the process of creating the installer USB and installing macOS from it will be faster that way. For macOS High Sierra, Apple recommends that you use a USB drive with at least 12GB of storage. The first thing you need is a flash drive, of course.
