Kiddies

The first years. Zuko, Katara, Sokka and Azula all meet as toddles because their parents knew each other. Katara and Sokka's mother dies when Katara is 4 and they live with their father for 4 years before he is sent to the war. They live with their grandmother after that (Katara is 8).

Teens

Katara develops a conscious crush in Zuko while Jet crushes in Katara. Zuko is oblivious to everything (as per usual). Sokka dates Yue and she dies from a disease.
Just before starting High School Zuko causes an accidental fire in his house that kills his mother and leaves him the scar. Ozai sends Zuko to live with Iroh.

Post Fire

Zuko is depressed and suicidal but with Iroh's help starts to get better, his social life is practically non-existant through High School. He loses a year of high school trying to cope with his mother's death. Ozai monitors all contact Azula has with Zuko. Katara writes Zuko but he never answers.
Iroh introduces Zuko to a friend's daughter, Jin. Her and Zuko start dating when they both begin their freshman year in the same college.
Katara is sad after Zuko leaves but learns to live with it, she outgrows her tomboy phase (a little ;) and starts dating Jet just before college (after Jet is dumped by Azula, who starts dating Haru).

College

Zuko, Jin: Sophomores
Katara: Freshman
Jet: Junior

Post College

I don't know yet... lol