In this new world of no more CD/DVD discs, you can go the route of preparing a USB memory stick with an macOS install image, but that always require access to another Mac for this, and of course a bit of searching around to find the correct image.
A simpler way of doing this, if you have access to an internet connection (a fast connection will good) is to fire the Mac into the Internet Recovery mode.
This can be done in 3 days, depending on what version of macOS you want to reinstall. And of course this will delete all data on the target disk, so be aware.
The complete guide can be found at the Apple support documents:
Use macOS Recovery on an Intel-based Mac
But in a short version taken from the above guide you can do the following:
1. Restart your Mac
2. Immediately press and hold one of the following key combinations until you see the startup screen:
- Command-R: Start up from the built-in macOS Recovery System. Use this key combination to reinstall the latest macOS that was installed on your system, or to use the other apps in macOS Recovery.
- Option-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall macOS and upgrade to the latest version of macOS that’s compatible with your Mac.
- Option-Shift-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall the version of macOS that came with your Mac or the closest version that’s still available.
Depending of the options you will need somewhere in the boot process to connect to the internet, either via a wired connection, but the simpler way will be via Wi-Fi, that you can connect from the recovery screens.
When this is done the Mac should start downloading the recovery app if needed, and then will allow you to choose how you want to install the fresh copy. Please refer to the Apple support guide above for a better explanation.