Cutting portion size (calories) and increasing physical activity are the only ways to lose weight safely. There is no way to lose weight in one area of your body by choice. You body is in control of where fat is added or used. Research nutrition and exercise. But a good simple start is to stop eating foods with added sugar: soda, candy, cereal, bread and pastries, etc.
Join a sport. Usually a very athletic sport. For a girl : softball. Soccer. Field hockey. Basketball. Etc. its the most fun and easiest way to lose weight. For boys: football. Soccer. Cross country. Track. Baseball. etc.
In football i used yo do theese things called "six inches". You lay on the ground and raise both your legs about 6 inches and hold it for about 45 seconds. Its a good workout for your legs and stomach so you should try it
I would suggest lunges and that one exercise where you lay on your back and lift your legs up and down without them touching the ground. And for the stomach I suggest planking and A TON of sit ups or crunches.