Well if it's for like a job or something like that you just need some clean pee, if it's for like paroll or probation they will make sure you really pee they'll go in there with you.
My friend had his dog pee for him before and he passed a at home drug test lol.
Or if you drink like green or blueberry tea it will help flush your system, but id take niacin pills and drink a shit load of tea, anti oxidants help filter the bad shit out of your body.
But since your test is tom then you should probably go to a few gas stations, some of them have shit you can drink or eat to make you clean for a drug test even just hours before you're getting tested.