It depends on the pill. Some are appetite suppressants and some are designed to speed up your heart rate. Some pills, like Alli, bond to fat you eat and make you have diarrhea if you eat too much fat.
At 16 it is not recommended to take any over the counter diet pills. Your body is still growing and these pills can give you permanent damage.
There are tons of easy exercises that will not only help you lose fat- but give you beautiful toned muscle so you can look sleek and fit. Not skinny but still flabby like you get when you only lose fat without building muscle.
Plus if you exercise, you can eat more and not gain weight. Obviously it needs to be healthy food.
If you want to lose weight then talk to your doctor, get some books from the library and research to see which plan you like best. You might even find one that your parents or friends would like to do with you. I really like Body For Life.