If you are from a visa-exempt country, you need an eTA.
Please check here to see whether you are from a visa-exempt country.
You can apply for an eTA online.
ETA (Etectronic Travel Authorization) Visa: Visa-Exempt Countries
Individuals from Visa-exempt countries have to obtain an electronic Travel Authorization (eTA). If individuals are refused eTAs, they are refused on grounds of the following reasons:
Criminality – if you have previously been charged with a crime
Misrepresentation – if you have misrepresented information to Canada immigration previously and received a ban for 5 years.
Previous Deportation – for overstaying the duration of your visit illegally
Medical Inadmissibility – you have a medical condition that either is a threat to the safety of Canadians or will excessive demand on health and social services.
Human Rights Violations – if you have previously served in the military for a country that has been deemed to have participated in war crimes
If you qualify under any of the categories above, it is important to contact a professional to determine the best plan of action for your certain situation.