For that question, explain that it's not that you want to be a girl, it's that you ARE a girl, and you're trying to make yourself seem like it to other people. At least, that's my understanding of transgender people, and I think I'm right? Anyways, I hope this helps!
In my situation (happily married and gay) I found the best way of explaining things was to be honest and speak from the heart. I didn't find this way easy, but it was the best way of talking to the people that I loved. To the people that are not understanding or disrespectful I'm just a bitchy smartass. I think it's a case of judging your response for your audience, How close are they to you, how trustworthy are they, how important is it that they understand you. I don't envy your journey, but all the trans people I've met have been extraordinary people. I assume you are equally amazing. (*hug*)
Just explain that it's not about wanting to be a girl, but that you have no choice over the matter, and you want to be able to express yourself as you always have been, and that you're tired of hiding the true you in the closet. Like YermanTom said, it also depends a lot on the audience. You have to judge what's appropriate at the time. Best of luck to you! ^_^
