When she first saw harry at station, she probably had a crush on him from there only. In Chamber Of Secrets, Harry was "rescued" by his friend Ron, and brought back to his home. When Ginny cam downstairs, she saw Harry sitting at the kitchen table, and ran out of the room. It is later stated by Ron that she was talking about Harry all summer.Later harry also saved her life in chamber of secrets thus she got attracted naturally. Love is something whose cause cannot be explained.