What to do if i have to submit a urinalysis for a job. how effective are at home tests?

Stop using drugs: Please forgive my frankness. The only way you can be confident of not having drug use detected by a pre-job urinalysis is to be clean. This isn't the place to debate the wisdom, especially with respect to cannabis. If low levels of the latter are detected, as they may be weeks after stopping heavy use, you can claim passive exposure credibly.