How could i lose weight? 14 year old girl?

I am a 14 year old girl, and recently i have become increasingly frustrated about my weight. At 5' 8", I am 120 pounds, and though I realize my BMI is low, I still do not look skinny. Definitely I am bigger than my friends, and I have large thighs. Over the summer I would like to get flat abs, and I would like to get overall more toned. Any suggestions on how I could achieve my goal?