The Government of Canada's official website to apply for an eTA. It only costs $7 CAD. Most eTA applications are approved within minutes of applying.
New entry requirement now in effect: visa-exempt foreign nationals who fly to or transit through Canada need an Electronic Travel Authorization (eTA). Exceptions include U.S. citizens and travellers with a valid Canadian visa. Canadian citizens, including dual citizens, and Canadian permanent residents cannot apply for an eTA.
Entry requirements for other methods of travel (land, sea) have not changed.
More information here.