• How to make your computer publicly routable on the Internet (open firewall or forward port)
• Basics of Linux administrating and terminal
• Basics how to register domain name and point it dynamically or statically to the machine

Use MDAD and save yourself a ton of time and effort. https://github.com/spantaleev/matrix-docker-ansible-deploy

• user names cannot be changed
• never delete your database, have backups, all the state is in there, if you lose your DB, you essentially have to set up a new server: Your server will say you are not in a room while others say you are, so you cannot join. Always keep your DB backed up.