Who decides if Canada goes to war?

The Governor General is technically the head of government of Canada and as such is the one who will officially issue a declaration of war if the case arise.

However, She/He cannot do so without the assent of the parliament.

In the case of the first world war, Canada had no choice in the matter. The declaration of war came from the imperial government of Great Britain and as a colony at the time, Canada was forced to join.

The treaty of Westminster of 1931 finally gave Canada the authority to handle its foreign affairs independently of the British crown.

As such, Canada formally, and on it own, declared war on Germany, Italy, Japan, Romania, Hungary and Finland during the course of the 2nd World War.

Since then, Canada has not declared war on anybody and all its military intervention were always made inside the framework of an alliance of organization (NATO and UN).

Canada has never in its history declared war unilaterally. (On its own, by itself and for its own benefit)