I have bingo wings, and i'm only 14 ... help?

My sister and mother said I have bingo wings. I can't argue with that, as I lost a lot of weight from being extremely overweight to being underweight , to being normal. Through all of this, I have developed some serious extra skin on my tummy , and under my arms . I look disgusting, and i'm getting a gym membership. What exercises can I do to sort this out ? I look very disproportionate :(