In the very top you can click communities. If you then click all you’ll see all communities that your server knows about.

• There’s a multi-instance directory of communities at https://browse.feddit.de/. Though communities that JUST got created may take some while to show up there… and it seems like a moderately large number of communities have been created over the last day or two.
• Another strategy is just to browse through community list at https://lemmy.ml/communities/. Being the biggest instance, the users there have subscribed to the bigger communities on any instances that federate with lemmy.ml so that’s a good way to find popular communities.