Watch what you eat: Lean meats, some carbs ( balance with your activity!! Carbs give you energy, but too much builds up fat), lots of fruits and veggies, and eat ALL THE TIME. Small meals, lots of times a day. Your metabolism will naturally speed up.
And excercise. If you don't want a lot of muscle mass, then use light weights or just body weight. Lots and lots of reps with those. I personally love to do as many reps in as short a time as possible, without stressing my joints and tendons. It works on my muscle explosiveness, ad one right is great cardio!!!