I think most has already been said, choose a factor/subject related to cannabis that you can study. but I would add that I think it would be wise to not focus solely on cannabis, choose a subject that you find interesting no matter if it's cannabisrelated or not, but that could also lead to cannabis.
for example, I am studying plant sciences. I could get into the cannabis-business if I finish it, but I don't know if I will, primarily I just like plants, all plants, so I would be just as happy working with plants that aren't cannabis. but I do have the option to get into cannabis, if the 'climate' is right when I finish my degree.(I don't really know yet what I want to do afterwards, maybe something in developing new varieties of crops focussed on organic growing, since that area is growing rapidly too, but maybe going into breeding isn't that much fun, since current breeding is a lot about genetics and techniques like markerassisted breeding, so a lot of data on computerscreens instead of directly working with plants).
(I first wanted to get into breeding after reading a book by Luther Burbank, wich I found really inspiring, but his work was back in the day when breeding was much closer to the plant, and genetics were just being discovered, with growing out massive numbers and observing for the right traits, but since then the techniques have evolved a lot)