I didn't watch the live action series and it's a long time since I saw the movie but as I recall the story was pretty similar but it just abruptly ended. There's a lot more story in the cartoon.
People hated the movie for a lot of reasons. Katara and Sokka were terribly cast, I think Aang was okay and don't remember much about the other characters. And after great animation CGI looks really unimpressive. Other things I've forgotten, I'm sure.
You should watch The Last Airbender and Korra before Seven Havens comes out.