Are you using any lemmy app right now? I’d recommend trying jerboa, atleast till the sync/boost apps are here in a few weeks. You can try to use the search function to find communities/subreddit.

You can think of an instance as a new reddit. So for eg, you can host a reddit called caboose and I can host one with my name. You can have your rules on what’s allowed on your instance. In addition, we can create a “subreddit” called memes on both our instances. You probably want to join the one with the most users. Assuming the meme community on the caboose instance has more users, I’d just join that. Once it reaches critical mass, that’ll become the default memes community on the fediverse! The people on my instance of reddit would just use the memes community on your instance since it’s all interlinked.

To start, I would suggest browsing via all. Joining communities with high user engagement for now. The niche communities will take time to migrate. In addition, when you search for a community, join the one which has the most users.