how to I initiate a backup?

Purchase an external USB3 drive (preferably SSD) that is 2.5 - 3x larger than your internal startup drive capacity. Using Apple's Disk Utility, erase and format that USB3 drive as either HFS+ journaled (not case-sensitive) with GUID partition scheme or as an APFS drive (again not case-sensitive). I name my Time Machine drives either Time_Machine or even TM.

Follow the instructions in Back up your Mac with Time Machine. It will automatically back up everything that changes between hourly runs. The first backup will be your entire startup drive contents and it will take some time. Never just unplug your Time Machine drive, but rather right-click on it and choose Eject "Time Machine". This will avoid drive corruption.