At CinemaCon, Disney announced that the first sequel will be called "Avatar: The Way of Water" and will be released on December 16th. The first trailer for Doctor Strange in the Multiverse of Madness is set to debut on May 6th, and we might have more proof soon.
The synopsis of the film is quite vague.
Set more than a decade after the events of the first film, Avatar: The Way of Water begins to tell the story of the Sully family (Jake, Neytiri, and their kids), the trouble that follows them, the lengths they go to keep each other safe, the battles they fight to stay alive, and the tragedies they endure.
The Way of Water isn't the only film to look forward to. Two years after the one before it, there are three more Avatars in the works. I'm not sure what to expect, but hopefully they're good, after all, director James Cameron once said the sequels will make you hit yourself with your mouth wide open.
One of the top-grossing movies of all time was the first movie in the series. I can empathise with you if you don't know much about the film, as Disney will be re-releasing it in theaters on September 23rd.