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There is an account called Fedi Follows on Mastodon that shares accounts according to their topics. Hope it helps.

What I did in the very beginning:

• don't worry about the instance that you are on. My server is quite generic and is meant to be a simple entrypoint to the fediverse, not a "community in itself".
• Go to the mastodon directory to find the list of servers based on interests. fosstodon is one for FOSS enthusiasts, beekeeping is for those interesting in gardening, mountains.social is for those that like hiking, etc. The important thing here is to find servers that are not huge, but that have an active userbase.
• Now, the real trick: for all these servers you found that could be interesting, you go browse at the federated timeline. This will give not only the activity from people in the server, but you will also see what they are following, boosting, etc. To me this was a way to find not just interesting people, but who the interesting people are following as well.
• Be very generous when browsing these timelines, and follow as many as you can possibly can. During the first week, try to add 50 a day.
• Follow hashtags. This will also make your timeline more lively and will bring different people who might be interesting to you.
• Write an intro post, and pin it to your profile. Usually pinned profiles are always visible between servers, so it will make it easy for people to interact with you.
• (Somewhat controversial, but I do it) If you have people that you like to follow on Twitter, follow them on Mastodon as well via a BirdSite mirror. You won't be able to do it from the larger instances, but if you join a more chill instance (like mine ;)) it should be fine.
• Evangelize: if you have friends on Twitter, ask them to join as well. Most Mastodon servers have a way to let you send invites to others.

When you start getting the feeling that you are getting overwhelmed, it's the time to cull down the list. Feel free to unfollow any person or hashtag that you think is too noisy or not really interesting.

You need to follow hashtags but primary groups. You will find groups boosting toots about your topics. It's the first accounts to follow then you will find others.

It's not that hard. People have to keep in mind, it's not Twitter and so it works differently.