Check out https://lemmy.world/post/9296, it explains several ways to discover and sign up for new communities. It’s more than a bit confusing without an introduction, but not to bad once you get the walkthrough.

There is browse.feddit.de.

It would be nice if this tool was available at other domains…

You can search on your instance in the search area: https://lemmy.ml/search

You can do a general search if you want or if you know the name of the community you want to sub to, you can search for !community_name@instance.url

Another great way to simply find a community is by using https://browse.feddit.de/ Search for an interest/community there and then do a search with the community address on your instance like I said above.