How to lose weight fast? ?

I want to lose 20 lbs. I'm 110 right now, and I wannna get down to 90. ( don't try to talk me out of it. I know my body.) I'm built small, but I want to lose the weight fast and effectively. I don't want to starve myself or anything. And I work out. Also,haha just wondering, I'm a C cup right now, if I lose weight how will it effect my bra size? (Im 21. Done growing.)