Selphie Tilmitt was one of the countless children left without parents at the end of Sorceress Adel's reign – and the consequent end of the Sorceress War – around seventeen years prior to the beginning of Final Fantasy VIII. However, she was lucky enough to be taken in by Edea Kramer, a good soul and the Matron of an orphanage on the Cape of Good Hope at the southwest tip of the deserted Centra continent
(no, not the one in South Africa. I think.). This stone house by the beach is where she met and spent her early childhood with Irvine, Quistis, Zell, Seifer and, of course, Squall, as well as the slightly older Ellone. Although already an extremely energetic social butterfly, it seems it was Irvine that she got along with best right from the start: at least, he does mention how "spunky little Sefie" was the most special person to him, and how she had the ability to make him feel happy just by talking to him.
At some undefined point, probably around the age of six-eight – unless she, like Quistis, was adopted first and it didn't go well, but this seems unlikely, because: a) she never mentions having had foster parents, and b) Selphie seems like the sort of child who would get along with practically any family, so I don't think it would have gone wrong –, Selphie goes to Trabia Garden, on the Bika Snowfield of the Trabia continent, in order to get military training: one of the few possible life options for orphans who, like her, had no other place to go. In her own words, and as could be expected from such a lively soul, she had "loads and loads of fun after going to Trabia", from which we can assume she had a smooth, light passage through adolescence (she doesn't seem too inclined towards heartbreak and drama, anyway; even during the game's events, when confronted with an indirect question about her love life, she replies she "just likes being surrounded by lots of friends"). When she was twelve, during outdoor practice, Selphie found a Guardian Force inside one of the monsters she defeated and, against Trabia Garden's policy, temporarily junctioned it: this is why she, like Squall, Quistis and Zell, retains no memories of her childhood in the orphanage
(in fact, she can't even remember the name of the GF... which is just... pathetic).
Being extremely talented in the hard manipulation of the nunchaku, Selphie decided to take the SeeD admission exam at the age of seventeen. After passing the written test, she transferred to Balamb Garden to take the (final) field exam, which was unavailable in Trabia: if you remember correctly, this is the day Final Fantasy VIII starts. She was one of the only four cadets (and the only girl) to successfully become a SeeD after the Dollet mission (the other three being Squall, Zell and Nida); from this point on, she becomes a constant member in your party, starting the very next morning, with the mission to assist the Forest Owls resistance faction in Timber. Unbeknownst to her, this also signified being reunited with her old childhood gang.
Selphie is the antithesis of Squall as far as socializing goes, and therefore had no trouble adjusting to her "new" friends and fitting in. She cares easily and dedicates herself very strongly to any cause she sets her mind on, which explains how and why she decided to stick with a group of near-strangers – or, more accurately, newly-found old friends (yes, strange expression, but how else can you put it? Damn those GFs!) – to the very end: let's face it, this goes way beyond just being loyal to SeeD duties and into the corny realms of everybody! love! and friendship! \o/ In the process, among many other things, she has to deal with her old Garden being
blown to smithereens destroyed by Galbadia (and the subsequent death of her friends there), develops a slightly silly crush on Laguna, arguably falls in love (thankfully not with Laguna) and gives "voice" to some of the most hilarious quotes in the history of video games. However, this is not the place to be analyzing, so I urge you to check out my other articles if you wish to deepen your knowledge a little.