The first place to start is the CIC website on this topic.
This site won't let me post the link, so you'll have to be creative here: www-dot-cic-dot-gc.ca-whack-english-whack-work-whack-index.asp (or go to the CIC website and follow the link for "Work Temporarily").
Note that unless you qualify for an "open" work permit or are working in one of the job categories that does not require an LMO, your employer will need to obtain an LMO ("Labour Market Opinion Letter"), which is a document that says you are not depriving an existing Canadian citizen or permanent resident of a job opportunity.
