If you’re totally new to Mastodon and the Fediverse, the safest easiest way to join is probably to go to JoinMastodon.org ⧉ or Fedi.Garden ⧉ and pick one of the listed servers. The servers listed there have all committed to specific standards of technical reliability and responsible content moderation ⧉.
You can find out more about a server by going to its website and clicking on the Learn more link on its front page (click the ⋯ button if you’re looking at the site on your phone). A server’s website address is usually the same as its name, so for example the server laserdisc.party would be at the address https://laserdisc.party ⧉
The server’s Learn more page will contain a summary about the server itself, as well as a list of server rules and which other servers it blocks. You can see how long the server has been going by clicking on the administrator’s profile link on the Learn more page; the joining date on their profile page tells you the server’s age.
You don’t need to join more than one server, because the servers talk to each other seamlessly.
You also don’t need to join a large server. The Fediverse’s servers are connected together, so whether you’re on a small or large server the audience you can interact with is the same. Small servers also have the advantage that the people who run them are easier to reach if you have any problems, and their “Local” timeline tends to be much more fun to browse (click here for more info about local timelines).
Also, whatever you do, don’t stress too much about which server you pick. You can always transfer your account to a different server if you change your mind.