I'm 15 and want to look better?

Yes, I'd like to lose weight but for my slow metabolism, losing fat means gaining muscle so I may not lose much. I'd just like to get a flat stomach and slim myself out. I don't really mind losing a bit of my chest cause I have plenty of it (I'm a 36D). I'm not terribly overweight, just about 156lbs. And I'm 5'3". Can you give me some of your best weight/fat loss tips to help me out? My boyfriend says I'm fine (sometimes he uses "sexy", even), but I still don't like my body.