Depending on your arrival airport and the airline that you are flying on, you will likely have to clear customs twice.
Incoming on the International side and outbound to the US on the US side of the airport.
You will have to clear Canada Customs when you arrive in Canada, then US customs to continue onto the USA.
You will also be required to collect your luggage to clear Customs, then check it through to your US location (usually just a bag drop after Customs inspection)
Unfortunately at this point you will have to move over to the USA departures area (usually in another terminal - but not always). You will have to go through Security screening again at this point before entering into the gate area for your US departure
Depending on the airport that you arrive in Canada at, you can have quite a hike from the International terminal over to the US terminal.
ie: in Toronto you will probably arrive in Terminal 1 and depart from Terminal 3
In Montreal all areas are in 1 very large terminal, however, arriving in one area (International in your case) would require you to collect your luggage at Customs and then clear Customs for Canada. Bag drop for connecting flights is just before you exit Customs. Once you exit Customs, you will be in the Public area, from here you will travel across the airport to the Transborder area were you will pass through US Customs, and Security Screening to enter the gate area for your flight to the USA
You will want a couple of hour layover (at least) to make this as smooth as possible.