Ginny is first introduced in Harry Potter and the Philosopher's Stone (she is, in fact, the first girl of about Harry's age properly introduced in the series). Her presence in Philosopher's Stone consisted of two cameos, at the beginning and ending of the book, both at King's Cross: A minor one at the end of the book where she points at Harry and squeaks "look, it's Harry Potter", and a more substantial one at the beginning of the book, where Harry managed to see her running, half-laughing, half-crying, after the train he was on- which contained her brothers, who were leaving for Hogwarts- then falling back to wave.
Ginny's entrance as a more solid character into the storyline co-incided with her entry to Hogwarts at the age of ten in 1992, a year after her brother Ron. While she appeared to play a mostly behind-the-scenes role during the events of Harry Potter and the Chamber of Secrets, placed in the school year 1992 - 1993- mainly looking mysteriously ill every once in a while, providing light comic relief resulting from her crush on Harry and showing insight as to Harry's opinion of his fame ("He didn't want all that!")- she turned out to be the key to the whole mystery outlining the year, as it was revealed that an old magical school diary made by Tom Marvolo Riddle, later revealed as Lord Voldemort, took advantage of her insecurities and innocence, causing her to spill her soul into it, and eventually started to spill some of its own spirit into her- possessing her to access the Chamber of Secrets and unleash the Basilisk within on the school.
It turned out that Lucius Malfoy was the one responsible for making sure the diary would find its way into Ginny's hands, and that his target had not been Ginny at all, but rather to discredit her father, Arthur, who had been trying to pass a muggle protection act. Riddle, however, put his own twist on the course of action he was predicted to take. Once Ginny told "Tom" about Harry's survival and the destruction of Voldemort, rather than using Ginny to attack muggles and Half-Bloodss, Riddle decided to force Ginny to walk into the chamber itself, so he may completely absorb her spirit and become alive again, and more importantly, to lure Harry Potter himself into the chamber. He was, after all, terribly curious about how Harry had "defeated" him, and obviously wanted to kill him as revenge. Harry indeed went down to the chamber, but with indirect help from Dumbledore in the form of Fawkes and the Sorting Hat managed to destroy Riddle and thus save Ginny's life. While not explicitly stated, this probably created a powerful magical bond between them, as Dumbledore explains in Harry Potter and the Prisoner of Azkaban: "When one wizard saves another wizard's life, it creates a certain bond between them... [...] This is magic at its deepest, most impenetrable, Harry."
Ginny was not involved in the storyline as actively throughout Prisoner of Azkaban and Goblet of Fire; she mainly appeared as background in various scenes, interacting with Harry a total of twice (catching his eye and laughing; being suggested as his date for the Yule Ball by Ron but, in an impressive display of integrity, turning him down because she has already accepted an invitation from Neville). She also stood up for Neville, demanding that Ron and Harry stop laughing at him, and when the dementors went aboard the train in Prisoner of Azkaban, she was notably affected by them as badly as Harry was (having acquired some gruesome worst memories to be forced to re-experience in her first year).
Her character was developed the most in Harry Potter and the Order of the Phoenix, where she was established as a kind character with a no-nonsense attitude, considerable acting and magical ability and a bit too much pride (refusing to accept help even when she clearly needed it). Apart from Phineas Nigellus, Ginny was the only one to directly confront Harry's constant inconsiderate, egocentric attitude and wallowing in self-pity, although she tended to do this in a much more diplomatic fashion than Phineas' elaborate speeches on how Harry's attitude was the precise reason he could not stand teenagers. Ginny was also used as a parallel to emphasise Harry's tunnel vision when he believed himself to be possessed by Voldemort; in one particularly ironic scene, during a whole ride Harry wallows in miserable thoughts of himself being the weapon Voldemort was using to attack people, "contaminated" and unworthy of the company of his friends- without Ginny having gone through the exact same thing in her first year occuring to him, even though she was sitting next to him for the whole ride. When she reminds him of this to make him stop alienating himself from his friends, it dawns on him that he forgot all about it and he genuinely apologises, something he was not inclined to do throughout his whole fifth year in general.
When Harry received his "life-long" ban from playing quidditch courtesy of Dolores Umbridge Ginny replaced him as Seeker on the Gryffindor Quidditch team, where she was successful, though not of the same calibre as Harry (She confessed that she would rather be a chaser than a seeker, and would probably apply for position as a chaser the next year, when the current three chasers would graduate and leave the team).
Ginny was a member of (And came up with the name for) Dumbledore's Army, a group started by Harry, Hermione and Ron to provide the practical instruction in Defense Against the Dark Arts, which Dolores Umbridge had removed from that course's curriculum. This, combined with her now being a close friend of Hermione, Ron, Harry, Luna and Neville and their expedition to the Department of Mysteries being the final trigger, made her a member of a more tightly-knit group consisting of the six of them, which can be viewed as an extention of the original trio (Harry, Ron and Hermione).
It was also revealed in Order of The Phoenix that during the previous year (Harry Potter and the Goblet of Fire) Ginny had given up on Harry ever reciprocating her feelings and turned her attentions to other boys, probably because she realised how fixated on Cho Harry was. Most notable among her romantic interests was Michael Corner, whom she dated for over a year and a half, but eventually broke up with due to him getting sulky over Gryffindor beatening Ravenclaw at Quidditch (in an ironic twist, Michael ran to Cho- Harry's fresh ex-girlfriend- for romantic comfort). It is made clear that in spite of Ginny's feelings for Harry being apparently gone her brother, Ron, is hoping to see them together still, and is not-so-subtly upset when Ginny throws him an off-hand remark about an alleged developing interest in Dean Thomas (whether such interest exists or was made up for the sole purpose of riling Ron up is unclear).
For reasons unknown, Ginny was fascinated by the egg in the Department of Mysteries, which kept hatching and un-hatching and was a part of the section the department used to study the concept of time.