I don't think marijuana should not be legalized (confusing sentences rock!), but I can throw out a few ideas why it shouldn't..
- Alcohol is already legal, and while it is a damaging substance, I think pot being legal would result in a lot of people using both at the same time. Right now, it's easy to get beer, and also legal. But if you want to drink beer, and smoke pot, you have to go more out of your way to smoke pot, and also risk breaking the law.
- It could open the door to the legalization of other, harder drugs.
- It would cause a lot of dealers to have to find real jobs as a result of their current jobs becoming defunct. Is this a bad thing? For the economy, maybe not; for the dealers, definitely.
- It's an irreversible decision, and if there are regrets about legalizing it after the fact, it's going to be very hard to undo the law after it's been enacted.
- Pot is reasonably easy to get now. If legalized, it would be even easier for those under the legal age to get a hold of it. I personally think if a kid wants pot, he will find a way to get pot, but the argument could be made that by legalizing the drug, it will become much more accessible to everyone, including underagers.
- If pot was legalized it could give our country a bad image globally. But again, I am not sure about that. From what I understand, pot is legal in Holland, and there aren't negative feelings about that country based on the fact that pot is legal there.
Again, if I had to chose, I would probably be in favor of pot being legalized, primarily because I don't care, don't use, and thus don't see the point in it not being legal. However, it's much harder to defend its illegality, than it is to preach for its legalization.
I had to take part in a similiar debate during one of my first semester speech classes, and I took the side against legalization. I'll tell you right now. You will not win a debate over marijuana before a class of students aged 18-24, regardless of how much you prepared and how well you performed. Our professor issued extra points to the group that won based on class votes, and as you can imagine, me and my partner weren't seeing any extra points.
It's harder to take the side against legalization, harder to find valid arguments, and much harder to convince others that it shouldn't be legalized, because pot is one of those things that is practiced by many, regardless of the legality. It's become a social norm, much like alcohol (but not at the same level, yet), and you probably won't convince people that it should remain an illegal substance.
As I look over my arguments above, they are weak and easy to refute. And they are honestly the best I could come up with. It's just a tough side to take, and I wish you luck. It's also tough to debate against your true feelings. I was in the same boat as you, we drew straws and despite my partner and I supporting the legalization of marijuana, we had to take the side against it.
It's much easier to debate about issues you are passionate about, and on the proper side, because otherwise the whole thing seems "fake," because you aren't truly arguing your point, but instead presenting your case as a formality that you don't necessarily agree with.
However, that can't be helped, so you just have to accept it and get the information as best you can. Again, I consider my arguments above to be valid (except for the dealer job loss one, that was satire), but they are also going to be very easy for the opposition to shoot down.
Ideally some of the other posters here can provide some better arguments for you, but I really think the debate over marijuana legalization is one that starts skewed in favor from the getgo.
Take care, and good luck.