Ask Jono: More Love Triangles!
Those love angles, remember? What if you're the one who's wanting someone, but they want someone else?
Oh boy, this question only begs more questions. Does the guy potentially like you at all, or is this purely a one-sided attraction? Does he have a chance with the girl he likes? Is he even a dude at all? Is this sounding too much like an interrogation? How did you know the victim?? Where are the stolen launch codes.
For the purposes of this post, I'm going to assume the following things: I have the genders right, you two have a moderately close friendship, and it's at least possible the guy could be interested in you. If this is the kind of relationship where you say, "Hey, future boyfriend!" and he says, "Who are you and what are you doing in my closet," you may want to skip to the end of the post, which deals with getting over him. Otherwise, I do have some advice, but it's the slow-burn kind of advice that takes effort, not the kind where you spread a rumor that the girl he likes has a false butt and he decides to like you instead. I can't help you drive these two apart, unless you get a job as a bus driver and one of them lives on your route. In this love triangle, I can only help you solve for your line segment.
Luckily, attraction isn't a fixed, rigid thing. For the second time in my career writing teenage girl advice, I am going to quote Hannibal Lecter, whom I think you will agree was a pretty smart guy, and one of our best cannibals: We don't seek out things to covet, we covet what we see every day. Sure, there's that love-at-first-sight kind of attraction, but also—and more enduringly—there's the kind that blossoms after you get to know someone more and realize there's actually something between you. (Note: I am speaking figuratively, and if there is literally something between you, e.g. a door or wall, it will be very difficult to date him.)
The point is, I don't know what effort you've made to win this guy over, but the fact that he has eyes for someone else isn't necessarily the end of the world. You can still establish yourself as a fun, easygoing person he'd enjoy dating; one recent study found that being fun-loving and playful are crucial to being date-worthy (participants ranked those attributes third and fifth in importance; physical attractiveness came in a distant ninth). I personally once fell for someone I'd been platonic friends with for years, and it wasn't because she grew a more attractive face on top of her regular face; it was because spending time with her was fun, and her slight flirting made my idiot brain gradually realize that she was, in fact, a girl. So turn on the charm, act flirty around him, and see if he responds; there's always the chance he thinks of you in friendly terms but his outlook would change if he realized you were interested (as a high school guy, he may not be aware of your interest, or of anything else that is happening ever).
That said, don't turn all silly or sultry if those behaviors aren't normal for you, and especially don't continue to hang around him like a sad but flirtatious puppy if he's not responding to your advances. If he's not interested, you have to get over him. How you do that depends on how close you two are; if you see him every day, you may want to distance yourself for a while so you're not teasing yourself with something that's not going to happen. Mope for a little bit, and then move on; it's okay to crawl under your bed and conclude that life is a butt, but then eventually you should go outside and remember that life is actually just life. Don't wallow in misery, or indeed in anything (mud, teenage girl tears, etc.). Just accept that this wasn't meant to be, and allow yourself to move on, to like other people, and to live the rest of your life; given the time, you'll get over anyone eventually.