I actually lost around 50 lbs in a few weeks back when I was a teenager just because I sat only in shorts infront of a mirror one day and saw how terrible that belly fat looked
I did it by eating like 1/2 - 1/3 of what I ate before and absolutly no more sweets and fat stuff(was like a crash diet, but I never put on any weight after that gladly)
You don't even have to do a lot of sport, just drink a lot of water and cut down all "bad" food like fast food, sweets, chips...
In theory it's really easy