This has been discussed a lot in the past (just do a search here on our forums on "CBD") but the problem is that the research shows that you need A LOT of CBD to make a difference in negative symptoms, and to take that level of CBD results in a cost of about $60 a day.
Also - cannabis / marijuana isn't the best way to get the CBD because it comes along with a lot of other chemicals and problems similar to cigarettes.
Anyway - feel free to try it, but don't get your hopes up too high. There are companies working on a medication based on CBD that will likely be more effective. See information on GW PHarmaceuticals.