Will I lose Weight by Fall?

ok, im trying to lose weight. i'm 13 and weigh 140 and i am five foot four. Im going into high school and I am looking to weigh like 125 obs. Im going to start by writing down what I eat for a few days then see what I can cut down on. I'm going to walk on the beach everyday (its like a 2 mile walk each way) I'm going to eat more whole grains and vegetables. Does this sound like it is possible to lose 15 lbs before September?