Upvote:4
Assuming you have booked a single ticket (as chx suggests in comments, you can enter the booking reference on aircanada.com to confirm this if there is any question), you can follow the "Connecting Guide" steps helpfully provided on the Toronto Pearson website by selecting "Arriving from International," "Departing to USA," and "Air Canada" for both the arriving and departing airlines.
You should review the detailed information there, but the basic steps are:
The good news is that, once you've done all that, you're all done with US immigration. You'll arrive at LaGuardia as if it was a domestic flight, and all you'll need to do is go to the baggage claim and pick up any checked luggage.
Air Canada has a (couple year old) video describing the process at YYZ, which might be helpful to you. There's a section for "Traveling to the USA from an international origin."
If there are delays such that you miss your connection, Air Canada will rebook you on other flights to New York. This could result in a significant delay to your journey.