Don't deliberately lose weight. Before the age of 25, the enzymes in your body will burn your fat, as long as you eat and exercise properly. This is also the reason why men tend to get fat after 30 years old, because the enzymes that burn fat will decrease after 30 years old. As for exercising muscles, it depends on which part you exercise. You can try to practice dumbbells on your arms for at least 30 minutes every day, because it takes 30 minutes for your muscles to really become active. Legs don't need special exercise, so it's important for you to walk to school or take fewer cars, especially if you are just 13 years old. Walking on your legs can not only make your muscles symmetrical, but also it is very important for you to exercise your hips, which will make your hips nervous, unlike some people (especially those wearing jeans) who seem to drag their hips to both sides. Now maybe you won't feel anything. But when you 18 or 19 years old and graduate from high school, you will find that your back looks great when you wear jeans or slacks. Girls like such boys very much. When you graduate from college and go out into the society, putting on the lining of your pants will make you very grateful for your efforts. This is very important, and the details determine perfection! As for abdominal muscles, I usually do sit-ups in junior high school with my feet above my head. For example, my lower body is fixed on the bed and my upper body is hanging on the edge of the bed. The first time 10- 15, after 7 days 15-20, 15, leave 25-30 every day.