once u get the work permit u go to nearest service canada center and apply for it...u get SIN number the same day and get the card in mail after rouhly 10 days
http://www.servicecanada.gc.ca/eng/about/reports/sin/applying.shtml
SIN is for worker to help them to establish credit history, getting pay checks and paying taxes. Student is not allowed to work . Once you get your work permit you can have SIN number.