Intro to mastodon.
https://joinmastodon.org/servers
Pick any server, browse the global feed Explore and 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.