If you are a new user to Lemmy, the most important thing is to subscribe to lots of communities so you get a nice feed of new posts and comments.
Click on Communities link at the top and select All. A long list of communities will appear. Subscribe to many of them. You can always unsubscribe if they are annoying you later.
This list of communities are only the ones that someone has subscribed to on this instance already. But there is something like 20.000+ communities in the entire lemmy network.
It's loading now so probably some temporary hickup. That site is not maintained by us at Lemmy.today but it's been a very good resource to link to so far.