Weight loss help please :)?

hello, i am a fifteen-almost sixteen year old girl and im around 5'7 to 5'8ish and i weigh around 132 pounds. One of my friends is getting married at the end of august and i am to be a bridemaid and i really want to look good in the bridemaid dress. The dress is very unflattering to my figure and the only way i think that i would be able to look good is to loose weight. I have two months to loose maybe 15 pounds? i think that would be good because i used to weigh 115 a year ago (i was the same height) and i was really happy with my body. So i have been hitting the ymca about 4 times a week for 2 hours everyday. i work out on the elliptical and according to it, i have been burning about 1100 calories everytime i workout(i set up the elliptical with my age and weight for the calorie count to be accurate).So i have been working out for the past 3 weeks and i havent really been seeing results except i think my stomach might have gotten smaller but it wasnt that fat to start off of with. The place i really need to loose weight is around my butt/hip/thighs area but i wouldnt mind slimming all over. I have been eating semi-healthily, like on an average day i eat 2 luna bars for breakfast, a slice of cheese pizza and an apple for lunch, a clif bar before i workout, and a salad and a sandwich for dinner. So now for my question, do you have any tips for me to help me loose the weight quicker? i havent noticed any weightloss and it has been 3 weeks so i must be doing something wrong :(. Do you know of some exercises i could do to slim down without bulking up my legs even more? and maybe a really good diet that is healthy and works good? i just really need to loose weight inorder to look good at my friends wedding, the rest of the bridesmaids are pretty petite and i dont want to be the huge fat one compared to them if you know what i mean. Thank you :)