Basically Mastodon is a bunch of different social networks, but you can follow/subscribe people who are using any of them. If you want your own server, you need to leave it on somewhere for it to pull messages from other people and push yours out. Otherwise, you can just pick a server and set up an account on it.

Which server you join doesn't particularly matter, but most of them offer some sort of "see all of the messages on this server" functionality, so if you pick one with a lot of people who have common interests, you'll have a better default experience.