You could use lemmyverse.net to look what instances and communities there are

I think it usually just involves look around the comment sections. People are like "I'm on this instance, it's good blah blah blah."

In my case though I chose lemdro.id because it's an instance that centers around a topic I'm interested in.

If lemmy.ml is accepting signups I heartily recommend it.

That's what she said