If your flight originated in Canada and you have a boarding pass for a connecting flight to a U.S. destination, you must follow the U.S. Connections signs.
You must then go through pre-boarding screening checkpoint “C”, and then go through U.S. Customs and Border Protection. Speed up U.S. entry processes by using the Mobile Passport Control app to submit your passport information and customs declaration and enjoy a priority line once in the U.S. zone. There's no need to worry about your checked luggage.
Note, however, that U.S. customs officers are authorized to recall your luggage, if necessary, for manual inspection. If the customs officers do not yet have the information on your luggage at hand, you will be asked to proceed to the waiting area.