Is it possible to loose 20 kilos before summer?

Hey, I'm turning 13 in July, and I really want to be looking fab for my birthday. I weigh around 80 kilos, and I'm really tall. I'm obese, but no-one notices cos, again, I'm quite tall. I've been on a diet for 3 days, and I haven't noticed any changes, except for one pair of pants being not tight. Can any of you please tell me how to loose all those kilos (44 pounds I think) before summer? Also I was never slim, and I'm really shy so if I loose 44 pounds, will that cut off all the fat that's making me self consious? Thanks.