Urine test cleanse

Ok so I'm new to weed and I need some advice on passing a drug test my mom is giving me. I'm 5'8" 120lbs and have a very fast metabolism. (I CANNOT gain weight) I smoked for the first time ever on the 9th of august with some friends. About a .4g I'm not sure though because we were passing a pipe. I smoked again on the 19th despretly trying to get high for my first time (no luck) but it sucked because I smoked a shitty paper joint I rolled that burnt so fast I could only get a few quick hits. Now i've been sober since the 19th but i need to be clean by the time my mom gives me the first check urine test. How long do I have to wait? I'm also a lifeguard so I sweat a ton and try to drink a lot of water.