As this is still a new server the communities you may want to join may not yet be shown on unilem.org. Use a site like lemmyverse to find your favourite communities. Copy the community id eg. [email protected] and paste in the search. Wait a few seconds and the community will pop up and you can subscribe. If you still need help DM me or reply in the comments.
Nothing I paste in the search seems to work. I've tried subscribing to Star Trek, Linux, KDE & Plasma Users, and Apple so far, and none of them work. It's possible I may be doing something wrong, as I'm not too familiar with the Lemmy system and how it works, having learned of its existence just half an hour ago.