If you've never been to Canada, you're admissible if you had only one conviction. If you get an expungement then you may be admissible after that without having to do rehabilitation work.
Of course, if you wait till Dec 18 then you'll be admissible regardless of any pot history in your record, with few exceptions.
In any case, you'd still have to mention this case to the officer when asked about any criminal record, even if it had no effect on your admissibility. If you have been to Canada before and did not disclose this history at the time on a visa application, then you may be inadmissible for the grounds elaborated on this thread which is called misrepresentation. It doesn't matter that weed becomes legal in that case, as it deals with your credibility.