I signed up on a smaller instance and only follow a couple of communities there, so I had to go out and search for things I was interested in. I pretty much just subscribed to anything that sounded remotely interesting, figuring I could leave later.
i mostly found stuff with browse.feddit.de and lemmyverse.net, as well as just going to the bigger instances and looking at the local lists for anything interesting. So I’m following communities across several of the larger severs - lemmy.world, lemmy.ml, beehaw.org, kbin.social, sh.itjust.works, lemmy.ca - and a few smaller ones that sounded interesting or relevant. Also fedidb.org is a nice tool to see info about the fediverse in general, including stats on Lemmy and Kbin servers.