To lose weight as an 11 year old girl...I would suggest eating salads before your meals along with a tall glass of cold water...this will fill you up and make you less likely to over-eat when it comes to your main meal.
Further, cut out most of the processed foods from your diet. Stick to veggies, lean meats, eggs, grains, nuts etc..
Also, don't eat until you're full, eat until you're just "satisfied"....when you feel you're at or near that point, stop eating for 15 minutes..it takes about that long for your stomach to tell your brain that it's full, so help your brain out by stopping when you feel you've had enough.
If you're still hungry after 15 min, you can eat a bit more, but this new practice should help you eat less.