Weight problems. I really would like some advice on what to do. I'm stuck. And changes need to be done fast...?

So I am a sixteen year old girl. Almost 17 and going into my senior year and my life is soon approaching. I'm and really overweight. I'm a vegaterian, I eat pretty well, if I eat fast food maybe once a month or not at all. My problem is exercise. I really need to shed alot of pounds. I used to walk around my complex, after dinner everyday. But now summer it's Sooo hot. Makes it unbareable. I dont have alot of money fir treadmills, or join a gym. But I want to make a change, I need to for my health. Diabetes, high blood pressure runs in my family. Please give me some tips.