If you are a citizen of a visa-exempt country and travelling to Canada by air, you have to request an electronic Travel Authorization (eTA). Go to this page to see how to get an eTA.
If you are from a visa-exempt country, you only need your passport if you are travelling to Canada by car, bus, train or boat (no eTA needed).
If you are not a citizen of a visa-exempt country, you will need to get a Temporary Resident Visa (TRV) before entering Canada. Go to this page to see how to get a TRV.