Civilian cooks are hired like any other public service employee. Check the following website for job openings.
http://jb-ge.hrdc-drhc.gc.ca/Intro_en.asp
I'm not sure if non-public funds organizations hire cooks (mess', etc) but that may be an option, would hold out much hope for it.
If you want to be a military cook check the recruiting forum for all the info (and more) that you would want to know about joining the army, that the first step, followed by your trades training and soldier qualification (if you're army).
Edit:
Look here for the official line on military cooks: http://www.recruiting.forces.gc.ca/engraph/career/tradeinfo_e.aspx?id=861