How many pounds can I lose and in how many months?

Okay so I need answers from good people not from those who just want to earn points. So the details are I weigh 89 kg (I think nearly 186 pounds), i'm 5.3' and my age is 15. I want to lose weight until my next birthday (22nd July 2009), but if I don't get my target I wan to continue. So I want to know how much can I lose in how many months. I will be joining Gym in January. The main thing is that hwo many pounds can I lose till Julyy 22nd or in how many months can I reach my target. I think of weighing 50-55 kgs :) Help needed