I think you just go to the top of the page and click “Create Community.”
Im brand new here as well, but my guess would be to start posting stuff about whatever your topic is, links etc so people who run across it have something to participate in. I imagine its kind of a labor of love in the beginning but if its not super niche I’m sure you’d show up on searches.
What are you looking to make? It may already exist on another instance and you can just sub there if it already has users.
thesanewriter