how can i pass a drug test?

i dont smoke weed that often anymore, but i have a long history of smoking every 2 or 3 weeks. i was never an everyday smoker or anything like that. i used to work out a lot, running lifting weights. but i stopped doing that for some odd reason when i stopped smoking. anyways, im 18 and when i graduate in mid june this year im gonna have a job interview probably the following week after. the guy hasnt said anything about a drug test but im pretty sure he'll have to give me one, its just the way things are around here. im gonna smoke probly this weekend. but after that im juuts gonna chill out and stop for a while atleast until i find out if its cool with my boss/ doesnt drug test me. Im moving out of my house in 2 weeks and going to live with my friend. so i'll be working out more often, and drinking tons of water. will 20 days be long enough to get the stuff out of my system with the way i will be working out and drinking?

if not can anyone recommend a detox or something like that? i really need this job, its the only way i can start becoming a responsible adult.

thanks!