eat healthy and exercise at least 3 times a week. AND drink matcha. it helps boost your metabolism. don't drink it at night tho if or you'll have a hard time sleeping as it also gives you energy.

There's no fast way . Any fast way is either not safe or will bring harm to your body . Its best to just go with the natural way like exercising and eat healthy food. I think its okay to gain some weight as long as you are not obese because being too thin isnt healthy either.

okay so..... maybe if you eat healthier and exercise more it'll make your metabolism faster or you ask your parents/ guardian or go to the doctor yourself and ask for pills that will help your metabolism.
but before going to your doctor I suggest trying to eat healthy and exercise more for a while and if that doesn't work go to your doctor.
I hoe I helped.

Fast weightloss isn't recommended. There is a chance you Will get The fat back because of yoyo effect. Starvation diets don't have good effects and you can get on the anorexic side. I think including more water, veggies, fruits and exercise in your daily life will be for the best. Also ginger tea and green tea activate metabolism. I like ginger tea but if u have heart problems you perhaps will have to avoid these teas. Limiting snacks will do the trick too. AND the most important thing is having a healthy mindset and a healthy home/work environment to encourage you to keep with the good work. I think that the psychological component is the most important. They say when there's a will there's a way. But there must be encouragement and less stress to nurture the said will.