My dad won't let me diet, but i need to lose weight?

I'm 16 and i need to lose anywhere from 10-20 lbs. it doesn't matter how fast, as long as it's less then 2 or so months. i don't want to hear about how 'my body is fine' and 'your too young' just answers please. it has to be discrete though. no diet plans or anything. just things like "drink water" etc. thank you.