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.
The most popular communities are available under All Communities. Just subscribe to whatever you think seems interesting as a first step.
There are also many more other communities on other instances, and you can subscribe to them from this instance. That is what federation makes possible, and Lemmy is a federated system. It talks to all other instances, gets new posts and comments and shares whatever you post or like as well.
Easiest way to subscribe to a remote community is to use a global community browser like Lemmy Community Browser.
To join a community: click on its name to go to the instance hosting it, and then click Subscribe.
You will get a message box asking you to enter from what instance you want to subscribe:
Enter lemmy.today and click Fetch Community.
Then you have to click Subscribe one more time here, and you will be subscribed. It may take some time for the server to get new posts from the community if nobody else has subscribed to it before.
And thats it. Enjoy lemmy!