The most unmistakable sign that a girl likes you is if her best friends give her up and spills her secret: She's into you.
Her friends might be breaking girl code, but they probably wouldn't share such a weighty secret with you unless they realized that you're also head over heels for their friend. Besides, it's always nice to know that her friends are on your side -- it's like they're giving you their approval to date her. This is the final and ultimate sign this girl is into you, so if you feel the same way, it's time to stop playing games and ask her out.
When it's time for the big question (whether that's asking for a date to prom or to see a movie) avoid text messaging, e-mail or Facebook, and, above all, don't use any cheesy pick-up lines. While pick-up lines can be funny, they're usually awkward and a major turnoff for most girls. Be yourself, and she'll dig your individuality.
Strike up a friendly conversation by complimenting her on making the dance team at school, or tell her that you noticed her ordering dinner at your burger joint the week before. Speak slowly and clearly, and look her in the eye, giving her your undivided attention. If there's a dance or some other school function coming up, this might be the perfect opportunity for you to ask her to be your date to the event. Who knows? Your first date could turn into a long-term, happy relationship.