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. !lemmy@lemmy.ml 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.
Some I searched for didn’t show up I believe this is because no one on the instance is is subscribed. I went over to lemmy.ml and searched for the community I was looking for and then copied the contents of the address bar into the search box over here on unilem and I could then subscribe hope that helps at all