The 20.04 Ubuntu Long Term Support release (Focal Fossa) officially supports the Raspberry Pi!
Running Ubuntu Server on your Raspberry Pi is easy. Just pick the OS image you want, flash it onto a microSD card, load it onto your Pi and away you go.
With handy installation instructions to boot.
Long term readers may remember that I put together a stack of Raspberry Pis just for fun one Christmas season. They haven’t been doing much since, but because I’m a heavy user of Ubuntu at work, maybe I can get more mileage out of them.