When flying from the US to Montreal, you may need to connect through a gateway city in the US whose airport has direct flights to Montreal. Such airports are listed here:
Unless you live nearby one of the gateway cities listed above, you will probably first need a domestic flight to that hub airport. From that hub you would then take a non-stop flight to Montreal to complete your flight from the USA to Canada. An international arrival airport in Montreal is YMX (Montreal International (Mirabel) Airport).
It is also sometimes possible to create an itinerary from a US city to Montreal which connects through an airport in another country, but for many travelers flying from the United States that may mean two connections and a longer flight. Hope this page helped you in planning flights to Montreal.