Mastodon I’ve found has a bit of a discoverability problem, but there are ways.

1 ) Start off with your local timeline: these are all the people that are on your instance as well. If you’ve chosen a “specialized” instance most of these people will have something in common with you: mastodon.gamedev.place for instance is filled with indie developers, mastodon.art is full of artists, and so on. The more general instances like mastodon.social have a lot more activity, but there’s no implicit link between people on it. It’s a trade-off: the more specialized of an instance you’re on the easier it is to find people like you and build a tight community, but the smaller the instance. The more general the instance is the more activity and people are on there, but less of it is relevant to you.

2 ) Go search up some hashtags of topics you like. For instance if you like baking go see what’s on #baking. If you’re interested in pictures of moss #mosstodon is great fun. If you like pokemon #pokemon, and so on and so forth. You can naturally follow hashtags themselves, but you can also try to use that to find people you may enjoy following - after all, if someone is posting baking pictures and you like baking maybe you’ll enjoy following them!

3 ) Go snoop out other instances. Some Mastodon clients allow you to directly view the local feeds of other instances, but you can always just go straight to the page of said instance. Find a few specialized instances for topics you like, scroll through the local feed for a bit, and follow people that look interesting to you.

4 ) Google: when I joined Mastodon I just googled a few people I like or followed on other platforms and saw if they had a Mastodon. There are also plenty of “Who to follow on Mastodon” articles out there.

5 ) In the “explore” feed you’ll find posts that are trending on your instance: often at times there are some good users there to follow, albeit it can get a bit “samey” if there’s a big news story going on.