I have three weeks to loose 12lbs. What should I do to loose the weight?

Ok so my brother is going to Afghanistan in like may and my mom wants to get family pictures taken around then end of April. In getting pictures taken my mom wants us all to loose weight.
I need to loose about 12lbs. But I love junk food, and hate working out, but I do love swimming.
My question is how should I go about loosing the weight? I dont really want to do something drastically and be really sore, but I do need to do something to get the weight off. What do you suggest I try to loose weight?
Also any foods I should eat or any foods I should stay away from? Also any exercises I should do that I can do at a YMCA?