Two airports serve Toronto: Toronto Pearson International Airport and Billy Bishop Toronto City Airport. Pearson is the main international airport, with a direct express train service to Union Station via Union-Pearson Express. The train departs every 15 minutes and costs around $12.
Billy Bishop is located on Toronto Island and a short ferry ride from Downtown Toronto.
VIA Rail and AMTRAK offer services from Canadian and US cities that will drop you at Union Station. Union Station is centrally located so you can easily find your way to your destination.
Coach travel is very frequent throughout Canada and some US cities. Megabus is one example, but provides seamless transportation to Toronto. Driving from the US to Canada is also a popular choice, but be aware that you may need an Electronic Travel Authorisation (eTA).