What do I need to know about going through immigration traveling out of the country for the first time?

From the US to Canada, assuming you don't have a connection, it will look like this:

Leaving the US there are no customs checks, it will be like a domestic flight, but you may go to a different terminal or arm of a terminal than usual.

When you land in Canada, you will line up to talk to one set of people before you can claim your baggage. Depending on the airport you're using, this may involve more kiosk action than people, but you will emerge either way with a small slip of paper. Then you will go with the crowds to baggage claim. Once you have your checked luggage (or once you've walked past the belt if you don't have any) you will exit through a small area where customs officers will want to see (and usually take) the small slip of paper and you will go out into Arrivals to get a taxi or meet your reception or whatever.

There is a small chance that during the first conversation, at the baggage belt, or at the final exit when handing over the slip of paper that you will be "sent to secondary" to have a longer conversation. When that is over you will rejoin the process, assuming you are let in.

Coming home, you will probably clear US customs in Canada. (There are no Canadian exit checks so you won't interact with Canadian customs or immigration on your way out.) In some airports, US preclearance is before you hand your checked bags over onto a luggage belt, but at YYZ they somewhat confusingly changed it so that you check in your bag and then go to US preclearance. Whatever, it's a similar process as entering Canada was. If you're bringing home souvenirs, and they are asked about on the form, say "yes." For example, maple syrup is food. A bored person will ask you "What food?" and you will say "some maple syrup" and that will be that. Do not fail to declare what you are bringing home with you. After the person you carry on to board the plane as usual. When you land you'll be in the domestic area and will simply claim your luggage and go home as always.

If you have a connection, you will clear Canadian customs at your first Canadian airport on the way in. You will pre-clear US customs at your last Canadian airport on the way home. Discuss with your airline whether you will need to interact with your checked luggage during the connection; it depends on the connecting airport.

Overall the lining up, talking to someone or using a kiosk, and lining up again can add anywhere from half an hour to more than an hour to the landing process. (Or to the departure process, meaning you need to arrive at the airport in time. Pay attention to the times they tell you to get to the airport, not just the boarding gate.) It depends how many other planes are there at the same time for landing, or how many other people are trying to leave for departing. A few times a year it adds 2 or 3 hours and then that makes the news.

Provided you have a Passport and no criminal history**, travel to Canada from the US is fairly routine.

Traveling both directions, there is no meaningful difference between carry-on only or checking a bag. This works just like any domestic flight. Use whatever your airline or other benefits allow and what works for you.

When arriving in Canada, baggage claim is after Immigration but before Customs. Immigration queues can vary widely but it would not be at all surprising if you clear Immigration before you luggage hits the carousel.

Neither the US or Canada have exit controls however, most major Canadian airports have a CBP Pre-Clearence Facility which means you will clear US Immigration and Customs in Canada before boarding your flight.

You are allowed to bring Maple Syrup home from Canada, if you purchase it before US Customs, you must declare it. Note, if you purchase it after clearing CBP at a Pre-Clearence facility, you're good to go.

Really, don't worry about it. Thousands of people fly between the US and Canada daily with 0 complication. Don't watch the great show Border Security until after you're home. ;)

**Canada and the US have a broad sharing agreement for criminal records.

Flying into Montreal, assuming no connections, you'll fly to Canada as though it's a US flight (except that your US airline will want to see your passport prior to boarding). Carry-on bags are the same as in the US.

In Montreal, you'll clear Canadian customs and immigration. (Be sure to declare all food! This applies going to the US, too.) Declare everything you're intending to leave in Canada, if anything. You'll be admitted and then you can begin your fun in Montreal.

Coming home, go an hour or so earlier than you would in the US as you will clear US customs at Montreal airport. You will land in the US as though you were flying domestically.

Travelling with a carry on in Canada/leaving: ensure your liquids are in marked 100ml containers, no more than 1L in a clear bag.

The last time I went to California, i had alayover from Toronto to LA to San Jose that took about an hour to leave the plane, passing security before getting to the next gate. My connecting flights were booked about 1.5 hours in between. Barely got where I needed to due to confusion of airport layouts.

Check the airport website but most recommend 1-2 hrs for domestic flights and 2-3 for international. Checking in early will speed things up.

Check what you're allowed to bring back into the US but you should be able to bring Montreal style bagels back. If you pick up prepacked smoked meat, the site states you should be able to bring personal amounts back. You can even confirm with TSA before you leave. Try the poutine while you're in the city.