A couple of notes and unsolicited advice as someone who is almost an old hand already...
(1) Your front-page will be more interesting as you subscribe to more things. You can subscribe to things from other Lemmy servers and they will be pulled into your feed here.
(2) Communities that are hosted on this server will show up under "Local".
(3) "All" shows all of the local content from (2), but also any content that this server had to fetch from other servers for others. Basically, when you subscribe to stuff, it'll end up in All for everyone else on this server as well. If no one on the server has subscribed to specific content from another server, it won't show up in All. As a result, All is sort of a cross section of our users' interests.
(4) If you were to sign up for another server -- say lemm.ee -- you would get a different Local and All. But you should be able to subscribe to the same things regardless of the server you chose.
(5) Some servers are not connected to others, for reasons. This is called defederation. It's basically a means to block an entire server who has a community not behaving in a way that doesn't jive well on your local server. Lemmygrad.ml is blocked from this server, for example. You probably won't notice, but on rare occasions you can't subscribe to a community on a blocked server.
(6) You can help the quieter communities grow by shitposting. Throw your backlog of old saved memes into them. There isn't as much traffic here as reddit, and the niche communities often don't exist (or are silent).
(7) Find a larger community to post to for engagement. For example, on Reddit I would subscribe to the WinnipegJets team sub, but on Lemmy it is too quiet. So instead I post my Jets content to the more general Hockey community so we can have some discussion. This will change over time.
(8) A good place to find communities to subscribe to is: https://lemmyverse.net/communities -- copy and paste the community name -- eg: !technology@lemmy.world -- into the search bar and then subscribe.
(9) Meow
(10) Try different sort options. New or Scales are my favourites.