The longer the message has to take the root the better. The younger the child, the easier they accept black and white ideas. In other words, get to them while they still think everything you say is gospel.
Make it an ongoing discussion.
A one-off talk won't do much good. Your child's brain and peer group changes, so your message should too. Appeal to your child's stage and personality.
Younger kids are often impressed by "the law's the law." Tweens may remember getting drunk can make them look ugly or do embarrassing things in front of friends. A young teen who's into sports may pay attention if you tell her that drinking could keep her from making the team. You may want to tell an older teen that having a relationship with a boyfriend or girlfriend is that much harder if drinking is involved. However you decide to approach it, keep it "of the moment." Kids, even seniors in high school, have brains that are less able than ours to appreciate long-term consequences.
Don't bother with scare tactics.
They rarely work, so don't force him to sit through a gory YouTube video or share the story of your co-worker's nephew who drove the wrong way down the parkway and is so is paralyzed at age 18. The reason? Our instinct is to disconnect from really extreme scenarios, to think, "Oh, that's not me! That's just crazy. Who would be that stupid?" Keep the conversation tied to those right here, right now, relatable examples.
The best way to feel at ease is to get the fact straight. If you don't know what you're talking about, you'll feel more uneasy. On top of that, if you, for example, say blacking out when you mean passing out, or insist "None of your friends drink!" when you can't possibly know that for sure, your kid will discount your message.
Another trick to make it an easier conversation is to avoid eye contact by chatting while walking the dog or driving your kid to practice.
Some parents note that teens seem to open up late at night, when younger siblings are asleep.
As with so many other situations involving your kids, consistency between both parents is key. If one parent is much more confident and comfortable addressing the issue, it's better to have that person do it alone than for the child to get mixed signals.