How can I diet behind my mums back?

Before you judge, Its just because Im 14, and in 85 days im going on a tropical holiday, and I'll be wearing a bikini, so I just wanted to slim down a bit. I told her, and she says to not be stupid, and she just disapproves, I managed to get her buy me some yogurt, so that when everyone else in my family has ice cream, I can have the yogurt. I do alot of exercise, but its not really helping me with my stomach area. Any way, how can I diet without my Mum knowing. I dont really want to throw any food out so how pleeeease help?