Intro to mastodon.

https://joinmastodon.org/servers

Pick any server, browse the global feed Trending (/explore), search for #hashtags or specific users, no need to join a server for that.

You need an account somewhere to follow hashtags and people, you can export your follows and import it on a new account if you want to change servers later.

Follow recommendations.

https://social.growyourown.services/@FediFollows

A hefty guide.

https://github.com/joyeusenoelle/GuideToMastodon/blob/main/README.md

Lots of links to other helpful stuff here.

https://fedi.tips/

For anyone reporting dangerous stories, take precautions.

https://ssd.eff.org/
https://coveryourtracks.eff.org/

Linux... I'd start with Ubuntu (supported more) and with experience move to Debian, the distro Ubuntu is based on. Debian is the most stable experience. You can match packages from less stable, fresher sources (testing repository, flatpak). Good to try an install on a virtual machine, like free virtualbox. I personally gravitate toward command line tools because they don't change their interface. It's setting things up how I like only once. Same reason to separate "/" and "/home" file systems.