Lemmy is a bit different for my friends on Reddit, and it can be intimidating to wrap your mind around. Let's talk about the big differences.
This is mostly another explanation of the documentation, which is found here
Lemmy is Federated
Okay, what is "Federated". Essentially it means that it is both one giant service, and many small services at the same time. If you're a Discord user think of Discord. You join a Discord server and in there you have many channels you watch. Multiple servers may have the same channels.
Lemmy is similar, but not the same. Lemmy has multiple instances (similar to servers), and in those instances we have communities like Subreddits. Take here for example, we have the community taylorswift.
However, that community is not only available to people on this instance. That community is shared (or federated) out to other instances as well, so no matter what instance you sign up for you can access communities here, and vis-versa.
If you've used Mastadon, it's the same concept.
Signing up
Signing up for Lemmy is as easy as any platform, but you will need to choose where you want to sign in from. You can choose any instance, including this one, or you can use another instance, like lemmy.ml or beehaw.org. Any public lemmy instance will be able to access content on this instance, so pick what works for you. To be clear, you can choose any instance and view content from any other instance from it, you will not be missing out because you chose one instance or another. If you're unsure you can probably safely choose https://lemmy.ml, however they have been extremely overloaded to to the migration from Reddit and are requesting to use other instances.
Long story short, if you like pop music and plan on spending a lot of time here, go ahead and register here :)
Accessing other content
As mentioned, you are not locked into the communities on this instance, you can choose any instance and subscribe to it here. I may have even already subscribed to a community you care about, just click on "Communities" in this server and you can see the existing ones.
If you want more, simply hit the search bar in the top right and search paste in the URL of the community you'd like to join. Once loaded (and it may not show a loading screen, there's a bug where it says "no results" give it a few seconds and it should pop up), then click "Subscribe". Again may take a couple moments while everything syncs between the servers, but you'll now be able to access your subscribed content here.
A great place to look is here. Just copy and paste any of the URLs into the search bar and click subscribe as shown above. (Fair warning, there's a lot of weird ones, alt left and right, just don't be surprised. Gotta scroll through those)
After a couple seconds it should show as Joined and be available to to you under "Subscribed"!