Polish citizens with biometric passports don’t need a visa to visit Canada as tourists. However, if they pal to travel to the Maple Leaf Country they have to have an Electronic Travel Authorization (eTA). Without it they could not enter the plane. To fill out the eTA you must have the valid passport and the email address. You also need to pay the fee of $7CAD. Here you can find the detailed instruction how to fill it up the application.
Polish instructions for completing the eTA application are also available.
You can stay in Canada as a tourist for 6 months if you don’t have any stamp in your passport indicating other date of your departure. The application to extend your stay you need to submit 30 days before the end of your visa.