We first meet Cho on the Hogwarts Express, at the beginning of Harrys 4th year at Hogwarts. Harry seems enmoured by her during a brief encounter with the lunch trolley. Harry is suspiciously picked as a Champion in the Triwizard Tournament, and he continues to admire Cho from afar - until the Yule Ball, where he blurts out his invitation to attend the Ball with him, to which she replies "I'm sorry Harry, someones already asked me, and i've said I'll go with him". Chos' date to the Ball was Cedric Diggory (d), who she dating right up til the end of school Summer. We see at the end of 'Harry Potter and the Goblet of Fire', when Harry returns from the maze with the deceased Cedric, that Cho was very upset that Cedric is dead.
We meet up with Cho early on in the 'Order of the Pheonix', when Harry is convinced by Ron and Hermione to give lessons on Self Defence. Cho is among many students to sign up for the 'private' classes and become members of the D.A (Dumbledores Army. The classes are private because Professor Umbridge, the New Defence Against the Dark Arts teacher and High Inquisador, has banned all groups of more than 5 people. Cho proves a willing Student. In the book, the student that told the Umbridge was Marietta Edgecomb. In the movie, that student is Cho Chang - the information being extracted from her by use of Veritaserum by Umbridge.
Chos' and Harrys' relationship falls apart, in the movie it's because she grassed up the D.A - in the book it's because Cho believed Harry would rather spend Valentines Day with Hermione granger than her. By the end of Summer term, they are barely friends.