You would have to look into which immigration classes you may be eligible for. First you should look into the provincial nominee program where you are studying to see if they have immigration classes for graduates. You could possibly apply after graduating with a job offer or after working a certain number of hours etc. If you find that you can not apply under the PNP, you'd have a chance to apply under Canadian experience class if you manage to work a full time skilled job for one year after graduating. If you get an off campus work permit during your studies, you can start working on that already full time after graduating while you wait for your post-grad work permit.
If you have not managed to apply for PR by the time you are about 6-7 months into your post-grad work permit, you should ask your employer to get an LMO for you so you can get a regular work permit as that will allow you to stay in Canada and work on other means to apply for PR.
