Air Canada is quite possibly the easiest airline to fly out of Pearson on. Make sure you print out your boarding passes the night before, and then you can skip the checkin at the airport. You can go straight to baggage drop and then on to security. Or if your like me and do carry-on only, you can go straight to security. We've never gone to Pearson more than two hours before our flight - but we park AT the airport. We flew Air Canada last April and our tickets even said to get to the air port 90 minutes in advance because we did online checkin. You'll need more than 2 hours if you're parking somewhere that you need a shuttle from. We arrived at 4:15am for this 6:30am flight, and we were at the gate waiting at 4:40am. 25 minutes from our car to the gate (again - we did online checkin, and skipped the baggage drop).
We flew out on Sunwing last week, with a 6:30am departure, and only left two hours. We were still at the gate with an hour to spare. And security was much more crowded than it was in April.
They are doing a new border procedure at Pearson which makes customs more electronic. Those "blue sheets" you normall fill out on the airplane, you now scan at the airport with a copy of your passports. So, when you are leaving - you scan your passports and then key in the information (where you're going, etc.) and then the machine spits out a receipt which you take to the border officer. So much faster. On the way home, you fill out the blue sheet on the plane and do the same process but this time you scan the sheet and then your passports. It will spit out a new receipt, and then a border officer will look at it and send you on your way. If you look suspicious, they'll send you to another officer who will ask you questions, but we were dressed in Disney - so we got through...
Like everyone else says - everything is pretty well marked. You go up for Departures, and come in on the lower level when you arrive.