Welcome! Really, there isn't all that much to know. Sometimes it can sound like a lot to learn because people tend to use the Japanese terms for things, so there's a lot of unfamiliar language being used. There are also a lot of traditions to learn about. But as far as the rules go, it's pretty straightforward: push the other guy out of the ring, or make them touch the ground before you do.

The Sumo Prime Time youtube channel has a lot of good videos with explanations about how the sport works, as well as highlights of past tournaments.