Whatever story you decide on, as an inside reference make sure it includes reference to the Escapist

. I actually have a lot of story ideas, but I won't tell you any because I'm a selfish git...
Just kidding. Seriously though, I do have plenty of ideas, but the best story will be from an idea you create yourself. That isn't to say you can't take ideas from other places. One of the greatest recent writers I've seen, our very own British guy Phillip Pullman, said this when talking about his His Dark Materials trilogy (note that it isn't the Golden Compass, sorry to rant but it needs to be known that it's a British film and story and should be referred to by the proper British title. /rant):
"I have stolen ideas from every book I have ever read. My principle in researching for a novel is 'read like a butterfly, write like a bee'."
Credit where credit is due, of course, but no story that I have read in a long time has been written without considerable factors being taken from other works of fiction, or indeed, non-fiction. Just make sure that you don't completely steal ideas, since creativity and innovation are important qualities in a writer and whatever story you come up with should still be your own idea. As long as sparkly vampires aren't involved. And I actually liked Twilight, apart from that one thing...
As a guy who tried to write before, in fact when I was your age myself (about 3 years ago), I can definitely say that you mustn't give up. I managed to write about one and a half chapters of a novel I attempted to create before giving up because I felt it wasn't good enough. Luckily, on my next project (a screenplay) I finished it despite having the exact same feelings. However, make sure that if you intend to do this, then don't give up whenever things go wrong. Just pick yourself up and start writing again, and things will turn out alright.
Before writing, as well, plan the story out and make sure you have enough of a story to fill an entire novel and yet not become boring. A lot of stories I've read got boring when it became apparent halfway through that the story wasn't thick enough for the book itself and the author dragged things out. For brilliant examples of authors who are able to bulk out a relatively short story to make books bigger, without becoming boring, I refer you to the excellent author Ian Irvine, although you may find his novels a bit too bulky and long (I know I did at first, although he is a great favourite writer of mine). Also, prepare a first draft of the manuscript then ask people to read it. Some companies will, for a fee, read through the manuscript and proof read it, and tell you problems, where you've gone wrong and what can be improved. No author has ever, to my knowledge, managed to produce a finished manuscript on their first copy. If you don't want to pay then ask friends, family, and maybe your English teacher to look at it, and they'll tell you what's good and bad. Only send it (a copy, NEVER the original version) to a publisher when you feel it's ready and good enough for print. However, the publisher may still reject it, so you'll probably be best sending it to many different companies just to be sure and to get the best deal. They're still businesses looking for a profit, after all.
That's all I really have to say on the topic. Apologies for the lengthy response, I could probably have made it a bit shorter, but the advice is pretty useful and found from accounts by other authors in interviews, forewords, etc. Good luck, and I hope you do manage to write a great novel. Plus, if you do, you can send everyone on the Escapist a free signed first edition copy xD.