Everyone is gonna learn best differently. There’s no best place to start.
Id start with solving a problem. For me, this was not wanting to make a backup to transfer my data from my old machine to my new one. So I built a little Ubuntu Server, setup a rudimentary samba share, setup users/groups, and figured out how to access that data from my network.
Docker is easy, you’ll learn it by mistake. It’ll haunt you like it’s some complicated thing until you realize you’re doing it and it’s literally incredibly straightforward.
From there, Id maybe say go to WordPress and follow instructions about setting up a WordPress site in a docker container. Oops, you just learned docker.
Id hold off on hosting email. I mean it’s a noble goal but it’s a fucking headache. But that’s just me! Like I said, everyone’s different.
Piece of advice, before you go hosting a monero server, dig into cybersecurity. Particularly server hardening. I recommend Hack The Box. There’s tons of platforms, though.
If you’re into developing and stuff, GitLab
Plex server
Jellyfin server
ELK stack or security onion
Get steam working, connect via steam link.