READ THIS!!! It is long, but it is worth it.
Okay, here is an argument that I just thought of. This will not be expected by anyone, and therefore will win you the debate.
You need to in essence agree with the person that you are debating, but use their facts against them. Here is how you will do it...
They will of course be talking about how Marijuana is a non-addicting, harmless drug. Rather than argue that, say that you agree. After a couple of seconds, with your opponent staring confused, begin your attack.
"Because Marijuana is a harmless drug, and everyone can agree on that, I can safely say that we would all much rather have our kids and others in society interested in it rather than more harmful drugs. Because marijuana is illegal, we have created a need and desire for the drug, and therefore a vast underground for people such as drug dealers to profit off of it. Most would argue that this is a bad thing, but I disagree.
If we were to legalize marijuana, this "pot underground" would disappear. There would no longer be a need for people to hide their affiliation with marijuana, and all of the profit would be sucked away from it... marijuana dealers would vanish. And everything would work out, for a little while.
The people who used to deal marijuana are not just going to fully disappear, they still need to make money to survive. So rather than dealing a harmless, non-addictive drug, they will move to dealing harder drugs. Marijuana will no longer be seen as something that is taboo, so there will emerge a new underground market for an even worse drug that is addictive, is harmful, and will eventually debilitate our nation.
By keeping marijuana illegal, we are supporting people's use of a non-addictive, non-gateway, harmless herb that keeps them calm and focused. If we make it legal, we will in effect support the use and profit from much worse drugs that we have no idea of the consequences of. "