You do not need a Study Permit if you enroll in a program that is six months or less and complete it within the authorized period.
However, as an international student you must obtain a Study Permit to study in Canada if you intend to study for more than six months.
If you are going to need a Study Permit it is recommended that you apply for one before travelling to Canada because applications can only be made from abroad. If you decide to extend your course or continue your studies for more than 6 months once you arrive in Canada, you will need to leave the country in order to apply for your Study Permit if you did not obtain one in advance.
More information on Study Permit requirements and getting a study permit can be found by visiting the CIC website: http://www.cic.gc.ca/english/study/study.asp