Just eat more grains, plenty of fruit and vegetables, and drink a lot of water.
Doing a colon cleanse at 16 is unnecessary and could even be harmful, since in most states, people doing those are unregulated and can have unsanitary equipment that will do you more harm than good.
Any weight you might lose emptying your colon is irrelevant since you would have lost it in the next few days anyway, by natural elimination.
So you might lose a pound or two but then you will eat for the next few days, and not eliminate so you will regain all the weight.
None of that weight matters anyway because it's not fat, or even muscle. Just waste.