OK, mine was more of a research paper but I also had to do a paper that was anti-legalization. Anyways here was my thesis statement:

"Today’s marijuana policy is what it is: It is based on lies and propaganda spread by the fear and greed of corporations and politicians, and completely overlooks the economical and medical value of cultivating cannabis sativa. Also, independent research has shown that the adverse health effects of marijuana have been grossly exaggerated in many government reports and documents."

I don't think you need to go over the history of it because your essay is more argumentative. So, you need to mention the economical benefits of growing marijuana/hemp, how it is cheap to grow and manufacture, the different things you can manufacture from hemp etc. Also mention the amount of money spent on the War on Drugs and mention how much it costs us as taxpayers to put marijuana offenders in jail. There are several states that have decriminalized marijuana to put less strain on the system. Going back to industrial production, there have even been houses made of hemp that have gone into production. There are a ton of economic benefits of marijuana, just pick two or three.

The next paragraph you need to focus on how marijuana is not as harmful as some people would say. Maybe you can try comparing it to cigarettes and how cigarettes are KNOWN to kill, while marijuana is not. Marijuana is such a great plant, really =P. It is less harmful than any of the OTC drugs you can get, and it really doesn't have many side effects. Critics will claim that long term usage causes permanent brain damage, but that is complete cow shit. There are medical studies and research that disproves this theory. ALSO, mention how marijuana is not the "gateway drug" that everyone thinks it is. Again there is research out there that shows no correlation between marijuana and hard drug use.

I think those are the two main points (economic/medicinal benefits) that you should argue for legalizing. Try to disprove the gateway theory because some idiot always claims that marijuana leads to other drugs even when you show proof that marijuana is really harmless. I reasoned that because marijuana is illegal and can only be acquired via illegal means i.e. drug dealers, that it is hard to regulate what drug dealers will have or not. Drug dealers don't really care about their customers, as long as they keep paying him/her for whatever product he/she can hustle. If marijuana was taxed and legalized, it could eliminate those drug dealers because they would have no incentive to illegally traffic those drugs. See what i'm saying?

Sorry if this is not really organized i just had a bunch of random thoughts and just spilled everything on the screen. if ya need any more help lmk