What is required for an American to get a work visa in Canada?

It's all here:

Essentially, you need a job offer from a Canadian company that has performed an exhaustive search for Canadian candidates for the job. This will include advertising, search firms, etc. Once that is complete, they can apply to bring in a foreign worker.

The exception is certain types of job designated by Industry Canada as high demand.

It's neither a simple process or a quick one.