Request your school to give you Letter of Enrollment or Letter or Registration stating that you are Full-Time Registered/Enrolled student. After getting this letter you need to apply for "Confirmation to Work Off-Campus" letter from CIC. You will make that request online by submitting "Case Specific Enquiry" from this link: https://dmp-portal.cic.gc.ca/enquiries-renseignements/canada-case-cas-eng.aspx?_ga=1.144760116.1079316027.1379171408
Select "International Student – Confirmation letter to work off campus" as TYPE of form, fill the required information, attach scanned copies of your Study Permit, Letter of Acceptance and Enrollment/Registration Letter (the one which is stated in above paragraph) and submit the form. You will get this "Confirmation to Work Off-Campus" letter within 10-12 working-days via email.
Now bring that "Confirmation to Work Off-Campus" letter which you will receiver via email from CIC, your passport and study permit paper to Service Canada Center and you will get your SIN.
All the best.