Well, to be specific she actually debuted at the end of the last season, but only with her face hidden by a hood. Here is the first look at Treme actress Danai Gurira who will provide the face, and character, beneath it..
Robert Kirkman, comic creator and exec producer on the show, explains how the live action version of Michonne will differ slightly from her comic counterpart..
“The essence of the character — her personality, her motives, everything that makes Michonne Michonne — remains intact from the comics. But like a lot of things on the show, there will be little tweaks and differences here and there. We saw that her introduction is slightly different from how it was in the comic, and her interaction with Andrea is really going to be a really cool addition to the character that I think will get television viewers up to speed, and they’ll get to know her a lot faster than comic book readers did.”