
That means you'd have to lose 40 lbs in just 3 months; I don't think you can do that safely. Safe weight loss is 1-2 pounds/week. The first thing to do is stop eating all junk food, such as chips, candies, sodas, pastries, white bread and regular pasta, etc.
At age 16 your body isn't fully developed, so you have to be careful of losing weight too quickly.