Im 16 and 120 pounds, and about 5"2.?

Well im not fat but as im getting older ive noticed that i get out of breath really easily (especially going up stairs!!!) and that i dont have a healthy diet at all by eating pizza rolls, junk food, and fast food all the time. I also dont really exercise at all, just walkin around school thats about it. But im starting to look at my body more and im noticing that im staring to get a lil bit of fat on the lower side hip/back part and that my thighs are not really shaped at all. They are just meat hanging there, im not sure if i have that ricotti cheese thing goin on yet though. My tummy is also not as flat as it use to be but i no if i keep it up this way i will end up gaining alot of unwanted weight. I know im a young lady growing into a woman and my body is supposed to change but i was just wondering if anyone could give me some health tips on how to keep my ok body healthy and fine like it is before it starts to get out of control. Thanx