I am 15 and want to tone my stomach and hip area...? (please read!)?

I am 15 years old, around 160-65cm in height and weigh 55kg. I do not want to lose weight as such, but lately the area around my stomach hips and lower back have gotten flabbier, and i want to tone ready for summer. I leave early of a morning at get home late at night, so it is hard for me to find the time to exercise, but i am going to MAKE the time! So i was just wondering what exercises would be best for those areas. I am going concentrate on eating healthier and i plan to walk on the treadmill at an inclination of 10, for 30min everyday... but i was wondering what is a good way to target those tricky areas such as hips, lower back and upper stomach (I think walking up hill will help tone the lower section of my stomach). Thankyou! (: