If you smoked for the very first time ever and didn't even get high.. there's a good chance that you didn't even inhale it. This will pretty much determine whether you could fail or not..

but as it is, I'd be on the safe side and start diluting. I'd say the day before your drug test, take some creatine and get that ball rolling.. next morning take some aspirin, drink an energy drink (anything with creatine in it), and then start loading up on water/gatorade. try to take your test after you've already pissed a few times, and just drink a little bit (not a lot) of water/gatorade between voids... don't want to get TOO dilute.

as far as legal trouble.. there won't be any. if you fail the drug test, the lab informs your employer-not-to-be and they revoke the job offer. nothing more.

on the outset, i think you'll have no problems passing. the most important thing i'd advise you to make sure you do is to at least not give them the first few pisses of the day..

good luck.