The best guide isn't written down: don't be afraid to try a few instances until you find one you like!

Once you're on Mastodon and following people, you'll start to notice the servers and gravitate towards the ones you like.

I would advise against mastodon.social since it is the biggest and also has some problems with moderation. But some people like it, so maybe it is good enough also.

To choose a server, check out https://fedi.tips/which-server-should-i-join-how-do-i-find-out-more-about-a-server/

The fedi.tips account also posts a lot of helpful tips and themed lists of accounts to follow/discover so I recommend following them too!