I know for myself, that I just gave it time.
After a handful of my teenage years, it became pretty obvious to me that I was bisexual, because I found myself continuing to be sexually and romantically attracted to both men and women, and wanting to pursue those relationships with people of both genders. As well, one of my earliest sexual encounters was same-sex.
Just an FYI on lingo while I'm at it:: sexual orientation, not preference, is really the word you're reaching for, I think. orinetation is the word used to describe what gender or genders we are attracted to. People who use the term preference (who aren't fundies trying to belittle orientation), tend to use that to mean who they choose to be involved with actively, or what practices they engage in. Or, to describe actual preferences, like say, preferring blondes over brunettes: as in, a blonde person is ideal, but that doesn't rule out a brunette. Whereas for someone oriented homosexually, being attracted solely or almost soley to women very well may rule out men. Make sense?
You don't have to come to any quick conclusions, by any means. Ultimately, I've found with myself and with most of my queer friends, and in my professional life, that for the most part, when you know, you do just know.
Sexual identity is something that develops over time, for the most part, so you'll find as the years go by, as you can really look at your sexual history, and your attraction history, it's a good deal easier to come to stronger conclusions. While at 12, and even at 16 or 17, when I did have girlfriends, I may have still not been totally sure where I fell on the spectrum, in my thirties, still with partners and attractions to all genders, there's really no question that I'm bisexual (though I prefer the term pansexual, myself).
So, don't feel hurried or rushed: sexuality, like most of ourselves, is a lifelong continuum. You don't have to name yours now, and heck, you don't have to name it ever unless you want to. Plenty of folks are perfectly comfortable stating that they love whom they love, and that's just that.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson