I have to pass a piss test in 3 weeks?

[Zjanshe]

So i have a Intake meeting in 3 weeks and i smoked yesterday bout half a g and i smoked last week tues. But i really need tips on getting this out of my system i plan on stop smoking till im older and when i think im more responsible.. but im 15 and 5'8 1/2 and very skinny like 130-140 ibs i have a very fast metabolism. i have drinked a shit ton of water today and plan on not eatin that much stuff for 3 days and just drink water. i dont know but i need tips on gettin this shit out of my system in 3 weeks please help... :/