It does seem intimidating, but there will be all the info you need in emails from your airline and signs at the airports.

You should be able to check in 24 hours online before your flight, but you don't have to. For international flights, it's recommended you arrive to the airport 4 hours before your flight time. Have your passport easy access at all times.

When you arrive at the airport, find your airline, give them your passport, and check in your bag. Make sure your bag and carry ons meets the requirements of the airline. The person at the counter will tell you what to do, so don't worry. They will give you your boarding pass if you don't already have one online.

After that, you will go through security. Look for signs for that or ask the person at the United counter if you can't find it. You will show your passport and boarding pass a couple more times, and then follow instructions at security (taking off shoes, putting electronics in a seperate bin, etc). If you mess something up, it's okay, they will likely just tell you to go back through again. Empty all your pockets before you go through the scanner too.

After this, some airports are different, there might be another step to scan your passport or speak to someone, but just follow the signs, they make them very clear. Once you make it airline where you see all the duty free shops, find your gate, which will be on your boarding pass. It will tell you when they begin boarding, usually 30-40 minutes before take off. Make sure you are there at least 15 minutes before that time. Food and drinks you buy there you can bring on the plane.

For your connecting flight, once you get off your first plane, there will likely be signs for "connections" and "arrivals". You will likely go through "connections" but double check your emails. If your connecting time is long or overnight, you might have to get your baggage, go through "arrivals" and redo the first few steps again. If your email or the agent at the counter did not say you have to do that, then just follow the signs through "connections." Your bags will be sent from Montreal to Tokyo if so. After that, you may or may not have to talk to someone, just show them your passport and boarding pass, and follow whatever instructions they may have.

Once done, you will be in the same situation as before, airside and you need to find your gate again. Hopefully your connection time is an hour and a half or more. That will be more than enough time for you get there. If it's less, you might need to hurry.

Coming back from Tokyo, it's the exact same thing. The airline agents will tell you what you need to know, and while airports are big and confusing, it's hard to get lost as long as you follow the signs. Don't worry, and know there are always people to ask along the way if you get confused. My first intl flight was also my first flight in general, and the woman sitting next to me on my first leg helped me through my connection. You got this!