Flash Pi OS Lite to a micro SD card.
If you like to install and update Webmin via APT, edit the /etc/apt/sources.list file on your system and add the line:
deb https://download.webmin.com/download/repository sarge contrib
You should also fetch and install the Webmin GPG key with which the repository is signed, with the commands:
wget https://download.webmin.com/jcameron-key.asc sudo apt-key add jcameron-key.asc
You will now be able to install with the commands:
sudo apt-get install apt-transport-https sudo apt-get update sudo apt-get install webmin
All dependencies should be resolved automatically.