Well hey there pal, it seems like you've found yourself balls deep in First Love with a Heterosexual Young Lady. Unfortunately, you seem to have the common predicament of also being a Heterosexual Young Lady.
Now, to clarify, this is perfectly okay and normal and in fact there is a rapidly growing trans community out there for you to help you better understand who you are and why that's acceptable (for more info Google Tumblr).
However I'm gonna assume that that's not quite what you were aiming for, which means that your first goal is to come to terms with the fact that a) she knows you like her, and b) she knows you haven't mustered the wherewithal to do anything about it yet. That puts you in a special position of blatantly being a weenie. As you probably guessed: that's bad. Not because there's anything wrong with being a weenie if that's who you truly are (and let's be honest here..), but because essentially no straight girl ever has been attracted to someone who clearly has no balls. That's not how being straight works, see?
So how're you gonna fix it is the next problem. The most important thing you're gonna have to realize is that you need to change who you are. Don't be afraid to, that's part of growing up, it's not like anyone in highschool really has a clue who they are in the first place. Hell, I'm 20 and I still don't know who I'm gonna be for the rest of my life. Change is nature, it's part of who we are and it's what the human race does to warp the world in their favour. It's time to start being a credit to that race.
First thing's first: stop trying to be her friend. That's literally the last thing on Earth you want to be is her friend. Be honest with yourself, you already KNOW you want to be her boyfriend, what horrible childhood abuse did you suffer to make you think that friend was an intermediary step to boyfriend? There's an easier way to summarize this, though I consider the friendzone to be a pathetic sexist excuse that men use to blame women for not liking them. So instead I'll give you the description according to most girls I know; think of it thusly: I want you to imagine your best male friend in your mind. Picture him, say his name to yourself, and then spend the next 10 minutes making out with him in your head. Go on, do it. It's not a lot of fun to imagine, it won't keep you awake at night in the "good way" (you and your kleenex box know exactly which way), and if you decide to enter a platonic relationship with this girl then that is exactly how she will feel about you and you will have only yourself to blame. That goes hand in hand with not ever doing nice things to win her over. Do nice things, but don't expect her to like you in return. That's not being nice, it's being manipulative, and I want to make sure you're aware of that before you fall into that trap.
The other reason you want to stop trying to be her friend is that if you try too hard you make yourself too available. That's not attractive, it's desperate which is the opposite of attractive. I'm not telling you to shun her, obviously, because only a 5 year old would fall for that, but don't go handing yourself to her on a silver platter. People prefer things they have to work for, and she's no different. Make her realize she doesn't have you in the palm of her hand. Make her work for you a little. Obviously, of course, don't just go for the relationship while you're unfamiliar with her, but familiarize in a way that makes it clear you want a relationship, not just a friendship. There's plenty of advice on the Internet on how to behave like that, and to read her behaviour to see if it's working. If you want to hop into dating hardmode, find a different girl who is generally recognized by the egotistical highschool community to be of even lower social standing than yourself and start flirting with that girl. Girl culture is rife with jealousy the way guy culture is rife with sweaty, half-naked tussles while loudly proclaiming our heterosexuality, and the girl you want will want what the girl you're flirting with has, which is you. Keep in mind that that last suggestion is a horribly cruel thing to do to everyone, but it is frighteningly effective and you are way more likely to get what you want (and also be a monster), and you still at least have someone to go out with as a fallback to help yourself discover that Highschooler's First Love is something you're going to mention with a chuckle at a social gathering five years from now and then cause everyone in the room to stare at the floor awkwardly and avoid eye contact out of embarrassment. Don't think I'm trying to belittle your feelings here or anything, it's just that nobody's harsher on our highschool selves than us.
Final thing is don't just be confident, be interesting. Organize fun events like parties or movies or even games of tag if that's what you guys are into (parties work best because of rum shots trust me). Be the guy that makes things happen. That's actually the entire point of all of this. If you're gonna attract a mate, you need to be so awesome that they want to affiliate with you, they want to be a part of how great your life is(/seems). Remember, fun things don't involve sitting around doing something boring, that's basically doing nothing but with a better excuse. "Video Game Night" is a card you may only play once, and that's if you REALLY wanna push it. Do things that involve being active and/or going out somewhere different and exciting. Having a lot of money greases this exponentially, so get a job even if it's somewhere ****** (you're in highschool nobody cares if you're president McDonald's (though that is pretty sad, seriously) as long as you've got spending money).
So once you've done all this and the time comes for you to make the move, one of two things will happen: 1. The stars will align, you'll put your lucky socks on in the morning, stroll over to her when she's all alone, carrying yourself straight, broad shouldered and comfortably, talk to her a bit about stuff no one cares about, and you'll ask her if she wants to hit up a movie and dinner afterwards, your treat, and you totally keep your voice even and don't break off into a mumble like a preschooler talking to Santa Claus at the mall, and she'll actually somehow agree to it and then you can celebrate and cry yourself to sleep in joy and go on many dates and become "Facebook Official" and ride the pony snake all the way to tuna town and then break up later when you start to slack off in your relationship somehow thinking you can just go back to being a boring weenie now that you've got the girl.
2. She rejects you and you go home and cry yourself to sleep and wake up and either slowly get over her, or...realize that in trying to attract her you learned a lot of interesting things about yourself and how to deal with people, that you became a way more interesting, self confident, destined for success go-getter champion of your gender and species, who has probably gained substantially more respect and social ground among your community of peers. On top of that, you'll have a job and spending money. You'll be confident, secure and ahead, and it'll make it that much easier to realize that you shouldn't ever need another person to fulfill your life. Women find all that attractive, and it's easy to see why. There's no reason not to be attracted to someone so worth knowing.
So that's that, my Berlin Wall of Text on how to Get The Girl. There's really no reason why you can't give it a shot. Why can't you change who you are? Why can't you turn your whole life around?
Why can't you be cool, like me?