To all whom smoke marijuana

Would like some help i smoke marijuana and the job i have now doesn’t do drug testing so i never really had to worry about it but i wanted to apply to a different job and they do drug testing i don’t know when they will do it the last time i smoked was the 19th which was when i applied for the new job. i haven’t smoked since everyone keeps telling me that i just need to drink water and it will flush it out but is it really true and if so how long does it take to get it out of your system? i also heard about a drink you take 15 min before the test but how do i know when i will be taking it ? should i take it right before i walk in for the interview? & it’s in july.