If you are staying in any hotels in Toronto, they usually have a shuttle to tour you around Niagara Falls and back to your hotels. Won't cost you that much and you'll surely learn a lot about the place while enjoying it.
There are limousines and buses available right at the airport as well or on the spot rent-a-car service if you'd like to drive by yourself. Make sure to pass by any of the Tour Desks (Kiosk) at the airport. Their service is free and will be happy to assist you. Goodluck and make sure you go for the Maid of the Mist at Niagara Falls. ---That's an awesome experience!