Originally Posted by ls17031

You'll need to change terminals in Toronto (from 1 to 3). You'll also need to rescreen through security and submit to US Immigration and Customs, all at terminal 3.

Go here to see the connections guide. You may also want to ask the Air Canada baggage agents at arrivals in Toronto if your baggage will be sent directly to Terminal 3 for you. Make sure to ask in Toronto, as agents at your initial airport may not have precise information.