As an 11 year old, you'll lose weight the same way an adult was:
1. Eat less
2. Exercise more
Now keep in mind when I say eat less, that is not entirely true. For instance, you can eat A LOT of vegetables such as broccoli or celery if you wanted to because they are very low in calories and do a good job of filling you up.
When I say eat less, I'm referring to most other foods other than lean meats, fruits and veggies.
One other tip to prevent over-eating is to eat slower and to stop when you're at about a 7/10 on the "hunger scale".
When you feel just "satisfied"...stop eating, don't stuff yourself.
Also, keep in mind it takes 15 min for your stomach to tell your brain it's full, so if you think you've eaten a good amount of food, give yourself 15 min before finishing off the rest of whats on your plate.
Another tip...drink a lot of water..sometimes when you think you're hungry, your body actually just needs water. Drink 8 glasses of water per day and you will find you end up eating less food.
There is a lot more info out there for you to learn in order to implement a successful diet plan/strategy. You're going to want to start reading up some more.
At the age 11 year old is not that much difficult to lose your body weight. Just make sure you doing right body exercise to burn body fat. Prefer those game, which is playing outside like football, cricket & volleyball etc.
Make a research for perfect diet plan because right diet plan plays almost 70-80% role in losing weight.