I am in the same boat (18 and trying to maybe let people know about it) but, like you, I'm not entirely sure how I would let people know (or at this point even whether I should let people know)!
I'm still kind of scared about it. My flatmate asked me (on the first day I met him, no less!) if I was a lesbian (I'm not, I'm asexual) but I realised later that he was probably picking up on my masculine mannerisms and was telling myself to go and correct him in the morning (sexuality vs. gender and all that), but... that never happened. It's a pretty hard thing to bring up in conversation! Plus I don't like some of my other flatmates and I'm not certain that they wouldn't be nasty about it.
My main problem (and the main reason I would want to 'come out') is that when people refer to me as female it really irritates me, and for some reason there are a lot of 'ladies first'-type people here. Which is fine if you are a lady, but I just keep asking them to not do it to me and they reply with something like "Chivalry is not dead!" >.< And the problem is, they mean well, but... aargh!

My vague plan at the moment is to get a binder when I figure out how my post works here and let it dawn on people.

@TooManyToasters: I know how you feel, I'm reaching saturation point with keeping how I want to be perceived bottled up. And I love your username, a fellow connoisseur of Hotel Mario by any chance...?