Have you taken a look at your instance’s FAQ here? Other than that, there are numerous guides to getting set up with Lemmy and how to subscribe to communities on other instances. It’s slightly different than Reddit, but apart from some technical details, everything works like Reddit once you’re set up.
Because of the decentralized structure, there can be communities on the same topic on different instances (with different subscribers, moderation guidelines, etc.). Use the search to find communities you’re interested in and post questions e.g. on the FAQ thread of your instance. Or right here of course.
Gird your loins, people running Friendica instances!