It had been talked about informally, but given the box office response to Captain Marvel (and Brie Larson's multi-film deal), it was a foregone conclusion that Marvel would make a sequel to the movie. According to The Hollywood Reporter, the sequel's officially in the works.
The news comes partly because Disney and the MCU team are busy hiring writer Megan McDonnell to craft the script. She has form with Marvel properties, as she's part of the writers' room for WandaVision, one of several shows that the company is making for Disney Plus. There's a connection to Carol Danvers' world as a grown Monica Rambeau features in WandaVision.
A bigger question right now is the director assignment, as it appears that Anna Boden and Ryan Fleck won't be back for a second go-around with the character (though they are in talks to make one of the MCU-adjacent series for Disney Plus), and Marvel is on the lookout for a female filmmaker to bring their vision to the new movie. As for what part of Carol's story will be featured this time? We'll see...