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.
Go to https://lemmyverse.net/communities in a new tab. If you want to subscribe to something here, copy the link (ex: !humor@beehaw.org).
Paste that link into the search box here at lemmy.today. Make sure Communities is selected in the list box to filter out comments etc.
It should look like this:
Clicking on the green link will take you to the community and you can click Subscribe on the upper right:
And thats about it. If you have any questions, just ask. :)