Step daughter came to us in trouble

First off, I think what you’re doing is beautiful. I was an overweight child (also in fostercare, so that went unnoticed) and I wished some one cared enough to put me on a diet. Instead, I struggled my entire insecure teenage years (battling fat after puberty becomes 10x harder because your body’s conditioned to grow at that point, not shrink) and became miserable. So what you’re doing is touching my heart. Now for the nutrition. DON’T go into this without reading up on it and understanding/researching weight loss plans and safety hazards. Don’t overwork her or pressure her, and never give unconstructive criticism.
Subtract 500 calories from her current daily intake, or if it’s really high or low, 1,500 is a good enough calorie deficit paired with exercise. Make sure she isn’t eating empty calories, she needs her nutrients and if possible, have her take a vitamin in the mornings. Flintstones chewables are a flavorful and popular choice among kids. By empty calories I mean starches, flour, sugar, candies, cookies, you know the drill. By full calories I mean wheats, nuts, veggies and fruits. Exercise is important and necessary to maintain weight loss. Try something fun, like tag or baseball/soccer/swimming/any sports classes. If she’s too shy to participate in groups, and you’re serious about this, you could buy her a treadmill/kid friendly elliptical.

Of course when you can, consult a doctor. She may be at the right weight for her height, or a diet might deprive her of some things she needs, she may need more than others, etc :)