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.

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.