Intro to mastodon.

https://joinmastodon.org/servers

Pick any server, browse the global feed #explore and search for #hashtags , no need to join a server for that.

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

Lots of links to other helpful stuff here.
https://fedi.tips/

The official guide, not very good in my opinion.
https://blog.joinmastodon.org/2018/08/mastodon-quick-start-guide/