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.

On most instances, yes! The instance owner provides server space for the community. When I was comparing instances, there were some that did not allow users to create their own communities so it looks that’s something the owner can choose to enable/disable. But no need to ask for permission if it’s enabled. I can’t find it now but one of the instances just posted a no porn rule on theirs. So you can always check your local feed to look for any relevant rules.

There's a step by step explanation on the Lemmy. World starting guide. It's stickied on this instance and easy to find

I think each instance will have rules on what kind of communities will be allowed and which kind wont be allowed