From Pearson Airport (YYZ):
The fastest and cheapest way into the city is the UP Express, located outside Terminal 1 in Pearson airport (follow signs for "Train to City"). Trains leave every 15 minutes and tickets can be purchased at machines on the platform ($9).
From Billy Bishop Airport (XXX):
Billy Bishop offers a free bus shuttle to Union Station, if desired. You're also pretty close to uber blah blahblah
From Union Station:
XXXXX
From Within the City:
Uber!