Any server running Lemmy software is an "instance". Each instance has at least one admin that sets the rules for that instance and adjusts its settings. The rules should be found in the "sidebar" (on desktop it's the panel on the right side of the screen, on mobile there's a button that says "sidebar" that opens it).

If you can create a community, them your admin allows it (they can turn that option off in their settings). From what I've read, Lemmy software is incredibly light so you wouldn't be taking up server resources by simply creating a community. The biggest issue seems to be how many active users an instance has, which is why most have a limit for how many users they'll register.