Well, you've already answered most of your question -- you apparently aren't very active. Walks are great, but at your age you should be running around and playing soccer and basketball and doing things all the time. You may not be able to get a flat stomach, it's not something everyone is genetically suited to have, and women in general need some room there to have children. Obviously, working very hard on your abs and completely cutting out simple carbs such as white flour will help, but most 15 year olds don't do that. Do you engage in any sports or athletics? And you don't really need to lose weight quickly, it won't stay off that way most likely. Anyway, if you tell us what your daily diet is like and how much activity you're getting and what it is other than walking, you might get some better answers, but nobody can guarantee you'll ever have a flat stomach, but you can certainly get a more toned one.
Oh also! My diet is on the healthy side and I don't drink soda ever or sugary beverages like juice, etc. I go on walks and stuff so I'm not sedentary but I need to do more.