Need help getting my body the way i want it?

Okay so I am a 16 year old girl I'm very athletic and play softball. I ca pretty much eat what ever I want and and don't gain any but I have cellulite and stretch marks on my legs and they are very "flabby" I only weight about 115 and am about 5'3 or 4 and I run 3 or 4 times a week I eat a lot of fast food so this may be the problem but, I really want to get my legs in shape and get rid of the embarassing stretch marks any tips on how to do this?