can some one help me debate against legalization on Marijuana?

That is a bad side to be on in a debate. Anyone who has taken the time to actually research can refute just about any of the reasons.

There are no good reasons to keep prohibition on marijuana. I can tell you how your arguments can be refuted though.

-There is no concrete proof it is a gateway drug. The gov sponsored studies are biased, in that they have never compared the number of people who use(d) marijuana but never went on to harder drugs and have successful lives to the number of people who did the opposite. The gateway theory came about by asking people in rehab for heroine and cocaine when their first illegal drug was. However, they did not ask about their first experience with intentionally getting high, (regardless of the legality of the substance), nor did they ask about other drug use that society typically does not view as getting high... such as a runners high, emotional over eating, etc.

-Does stoned driving really harm other people? Some studies demonstrate that people drastically under the influence of marijuana drive better than those who have had a drink or two, but are still under the legal driving limit. In addition, the gov's own studies report that there are more people on the road any given day who could be legally charged with dui from marijuana than alcohol. however, Marijuana accounts for less than 1% of all dui charges across the country, while alcohol accounts for 95%.

Nope, and whoever you're up against will smack you down.