One possible plotline could be that Ginny is jealous of Harry's fame and starts to resent him for always being in the spotlight. Another could be that Harry mistakes Ginny's actions for something malicious and they start to dislike each other from there. Maybe Ginny blames Harry for something that happened to her family during the war, even if it wasn't really his fault.
Well, a plotline could involve Ginny being influenced by dark magic in a way that makes her oppose Harry. Or perhaps Harry is forced to take a side in a wizarding political issue that Ginny disagrees with strongly, and this leads to their hatred. It could also be that there was some kind of misunderstanding between them during their school years that was never resolved and festered into hatred over time. For instance, if Harry thought Ginny was the one who spread a false rumor about him.