It's illegal. The trouble should you get caught, may not be worth it. But as far as that goes, just don't get caught! Haha.
Weed isn't bad. It's all natural. Honestly there's no reason for it to be illegal. I would say go for it, if you're doing it for the right reasons. Don't smoke to fit in or come off as 'cool'. Don't do it to escape reality. Do it because you enjoy it and genuinely want to do it.
Smoking once a week or once a month will NOT hurt you. I used to smoke once a week and for me, that was just my relax day. It becomes a problem when you start smoking daily and blowing every dollar you earn on it.
And the above poster is right, about how it's sometimes a gateway drug. Sometimes people smoke it, get curious about other substances, and try those. Just remember that weed has no ill-effects, nor do shrooms (when taken within reason). But others like LSD, Molly, PCP etc, do. Those are chemically made and will fuck up your brain.