You might be coming to Canada for an internship opportunity. Internships require a work permit (see the “You have a job and need to relocate” section) in addition to a study permit (see the “You want to study” section), because you will be working as well as studying. You must apply for the study permit first.
Once you have your study permit, there are certain conditions that must be met before you can apply for your work permit. You must be able to demonstrate that working at a job is a necessary component of the academic program you have been accepted to, and that you will not be spending more than 50 percent of the total program time working.
For this kind of work permit, you won’t need to pay any additional fees. Complete instructions on how to apply can be found here: http://www.cic.gc.ca/english/study/work-coop.asp.