Let's see...
You already have backup of all your data, right?
Here's what you should do:
Still inside your already installed system, download Catalina from the Mac App Store.
Get a pendrive and format it MacOS Extended Journaled (HFS+) using Disk Utility
Once the installer is downloaded, you'll have a macOS install.app (something like that) in your Applications folder, so use this command here https://support.apple.com/en-us/HT201372 to create a bootable drive.
Once your bootable drive is ready, restart your Mac holding Option, chose the pendrive and you'll be directed to the Recovery Mode.
In there you open Disk Utility, Click View and select Show all your devices, find the internal drive of your Mac and format it (Remove even the partitions, create them again from zero). Assuming you're going to use Catalina, APFS is best. Once it's done you can close Disk Utility.
Back to main window in Recovery Mode, click Reinstall macOS and follow the instructions.