Your best bet is just to go to the post office and tell a clerk you want to mail that to Canada, and they'll handle everything.

The most important thing to know if you're mailing a package (a simple letter may not be as big of a deal) is do NOT choose the cheapest option. I made that mistake once without even thinking about it, and that package was never heard from again. I heard rumors that the cheapest option involves throwing the mail on the back of a hay wagon and hoping it gets there eventually, but I have no proof.