Depends on your POE. If your landing through Toronto, you'll have the option of getting the SIN number immediately after you complete landing formalities. I'm not sure which other airport has a desk, you need to some research on that.
Otherwise, you'll have to find the nearest Service Canada location, walk in, and submit an application - should take a few minutes, but you should leave the location with it. Also, in this case, be sure to carry the relevant documents with you.http://www.servicecanada.gc.ca/eng/sin/apply/how.shtml
