There are a number of ways to get from Pearson to the city, but most convenient way is probably Uber
Uber has 3 options departing from Pearson now:
Uber Black and Uber Select are the two premium service options. Uber Black is operated by actual limo operators and is usually a black Lexus sedan or (in few instances) a Lincoln Towncar. The price is around CAD$90.
Uber Select is operated by individual drivers instead of limo operators, and there are a bit more variation in car models. I've ridden Lexus, Lincoln Towncar, Toyota Highlander, Chevy Suburban, GMC Terrain. The price is around $70.
The last option, and only started recently, is UberX. This is the cheap version and drivers are basically any regular Joe with their own car. The price is around $40.
How to use it:
Make sure you have Uber app on your phone and you already have an account set up before you come to Toronto
When you're in the terminal, connect to the free wifi at Pearson (the network is called Toronto Pearson Wi-Fi)
Open the app and get a car after you clear the customs and have your luggage with you. The luggage pick-up in Toronto takes an ungodly amount of time (plan 30 minutes to get your luggage)
There are designated pickup areas for each of the 3 options. The Uber app will tell you exactly where you should meet your driver
In terminal 1, the pickup area for UberBlack and UberSelect is Door A, which is on the same level as the arrival door (meaning you don't need to take an escalator up or down after you leave the customs area). The pickup area for UberX is Door Q, which is on the ground floor, one level below the arrival hall. You'll see a sign outside of door Q that says something like "Ride App Pickup Area'
In terminal 3, the pickup area for Black and Select is Door A, column 9. For UberX is Door D (once you leave the terminal building at door D you need to cross the street and follow the signs for Zone 3). In terminal 3 both of these areas are on the arrival level so you don't have to go up or down
Once you leave the terminal building you won't have wifi access anymore, so I recommend staying inside the building until you see the driver pulling up to the pickup area