Citizens of Western European countries and the US usually do not require a visa to enter Canada. Citizens of most other countries do. A list of countries whose citizens require a visa to enter Canada can be found here and information about obtaining a visa can be found here.
Pierre-Elliott Trudeau Airport (Dorval/Montreal-Trudeau): Located on the western part of the island of Montreal and 15km from Downtown Montreal, it services various airlines and is used for both international and north American flights. You can visit the Aeroports de Montreal website to get a list of those airlines or for more information on Dorval airport. Fares from the airport to Downtown Montreal: