I smoked a bowl about two to three weeks ago and I have a drug test tomarrow two days ago I failed one how can I get weed out of my system today?

Sorry. Marijuana will probably show positive for about a months in most cases, no matter what you do. I don t mean to sound judgmental at all (or something else)...But the best way to get a negative drug test is to learn to say no to drugs. Come on, you know better !
You can't really. The solution to pollution is dilution. That means, while you cannot get the weed out of your system today, if you drink lots of fluids prior to the test, your chances of testing positive will be diminished. Also, it is a little hard to believe that a single bowl two weeks ago (three weeks is 50% longer than two) lead to a positive test. I suspect you have been smoking more, but i could be wrong.