Diet and exercise go hand in hand. If you have problems with the diet portion, join a group such as Weight Watchers. My nephew is 12 also and has just lost 45lbs on Weight Watchers. There are a lot of fad diets. Don't do them. Usually what happens is that you lose the weight and start to eat normal again and your body stores more fat because it thinks you're going to starve it again. This causes you to put back on your weight and possibly more.
A healthy diet and exercise is the only real way of reducing weight, increasing strength and being healthy. Don't forget to to cardio exercise or sports. Even if it's just brisk walking around the block or on a treadmill. Moving around will help. If possible, swimming is also a very good way of exercising. Especially if you have muscle and joint problems.
It depends how tall you are. 140 might be quite reasonable. But if you are only a bit plump, you are only 12 and still have growing to do. Don't try and lose any weight---don't diet. You will "grow into" your weight as you get taller.
There is no need to diet (unless you really do need to lose weight. When you are only 12, this is the time for you do fun activities outside. You have plenty of time to diet, but for now you should just be a normal kid who is having fun. This will lead up to a good health plan.
If you need to lose weight, eat a healthy diet, make sure you are physically active (walking, biking, swimming, playing other sports) and let your height catch up.
For more infomation, see the related questions and answers, further down this page, listed under Related Questions.
