There are a lot of anti-drug messages out there - but most of them come either from people who never used drugs or have addictive personalities to the point where if common drugs didn't exist they'd end up addicted and homeless due to orange juice addictions.
So, I feel like there should be advice out there from someone who isn't a moralist, did do drugs, didn't have major problems with them but also figured out that it's best to just stick to booze and cigs.
Here's why you probably want to avoid most common drugs:
Pot/Hash: Being lazy and eating lots of pizza is no way to live a life. Plus, it will make you think really dumb shit is funny. Do you want to contribute to Adam Sandler becoming rich and famous?
Cocaine: First it will help you become less shy. And, you'll get laid more. And, you'll be able to stay up later and stay skinny more easily. It seems awesome. But, really, you'll quickly become an egomanic asshole, spend most of your savings, lost empathy for people around you and shit all the fucking time. Eventually, if you are female you'll end up having sex with guys you don't really like just because they have coke. If you are a guy you'll end up fucking girls who hate you just because you have coke.
Meth: Is it really worth becoming a toothless psychopath just to be wide awake during the late shift at your Wal-Mart job?
LSD: Fun for about two hours - but you'll end up tripping for like 12 hours and you'll be really fucking tired of it long before it's over.
Ecstasy: This drug will give you the shittiest taste in music ever, make you want to walk around with a fucking pacifier in your mouth so you don't grind your teeth and make you act like a wall hugging hippy. Plus, you'll become friends with a lot of douchebags.
Shrooms: Go to your favorite bit torrent site right now. Download any random Phish album. Ask yourself if six months from now you want to actually enjoy that type of music.
Heroin: The problem isn't that it will kill you - it's that it will take decades to kill you and during those decades you'll become the shittiest person known to mankind.