I also agree with CB above. Spell check would be great. It isn't too bad, but a once-over and it'd be much better.
Also, I'd question the reality. I know it's fiction but, if you got an envelope with that note, would you actually do it? I know, for a start, I'd question it. I'd probably throw it away and laugh it off as a joke. Then, if I got another note which said a similar thing, maybe I'd start to take it seriously.
I like the ideas you've shown so far, but slow down and pace the story out a little. I know you might just want to get to the core of the story (as most people do) but, sometimes, the slow set-up of events is what makes it. Suspense and tension is annoying but it's the key to a great story.
Also, I don't care much for your character. All I know is that you're 16. Introduce me, tell me more about you. I know more details will probably arrive as the story unfolds, but a small introduction at the start (or in your first few paragraphs) would help readers latch on and partially relate to the character.
Hope you take some of my ramblings on board xD
But I look forward to more
