Check out this youtube playlist. The woman speaking is from the Marijuana Policy Project and she brings up some interesting facts and statistics about marijuana studies and some other shit. https://www.youtube.com/view_play_list?p=37C27950D1598E43
Pretty basic, but still good. Regarding the "Gateway Myth", I always say anyone who desires to try more drugs already has that opinion in them. It's the reason why they decided to try weed in the first place. That's what I say to my mom because her constant arguments are that it's a gateway drug and i makes you a couch potato. Sometimes she'll mention crime/danger. I'm like Mom, weed doesn't make you want to kill anyone or be abusive. Then she says "Well, probably because they don't want to get off the couch."