By Canadian convention the Liberals will be given the first chance to govern even if the Greens (or NDP) don't prop them up.  They will be allowed to govern until they are defeated in a confidence motion.

If/when the Liberals are defeated in a confidence motion it is then up to the Lieutenant Governor's discretion to decide if she wants to allow the NDP a chance to form a government (including the option of forming a coalition with the Greens) or if another election should be called.

Long story short, there is no guarantee that the NDP and Greens will get a chance to form a coalition, and even if they do such coalitions have had a pretty rocky history in Canada.