Well if you're on the lemmy.world default web interface there should be a "Create Community" button at the top of the page. I never used it tho.

If you want to look for existing communities, check out the Lemmy Explorer. Apparently there are several existing MMA communities: https://lemmyverse.net/communities?query=martial+arts One strategy is to take over an existing community: 1) contact the listed mod, 2) if they don't answer then contact Support for that instance and ask to be made a mod. But if you end up just making your own community, then you can make posts announcing it on all those old communities.