There are 3 international airports in San Francisco - San Francisco International Airport (SFO/KSFO), Norman Y. Mineta San José International Airport (KSJC), and Oakland International Airport (KOAK).
Terminal 1 (Boarding Area B), Terminal 2 (Boarding Areas C and D), and Terminal 3 (Boarding Areas E and F) handle domestic flights (including precleared flights from Canada). The International Terminal (Boarding Areas A and G) handles international flights and some domestic flights.
Most international flights depart from SFO's International Terminal. Please note that Air Canada is located in Terminal 2. Check-in desks are located on the Departures Level of the International Terminal Main Hall. Be sure to check the requirements for your destination—as well as any transit points.