How can i loose weight and look amazing in a bikini in 3 months?

I am 13, My heights 5"3 and weigh 145.I have been on a diet and everything and i GAINED 5 pounds, i don't like soda and I've been eating very healthy with small portions lately. I chew my food slowly so i get full faster, PLEASE help answer my question, if you guys are wondering why i want to look good in a bikini is because im going to aruba in july and i want to feel great and not self-consious about my gross fat. PLEASE im begging for anyone to help!