All persons traveling by air to Canada and are citizens of one of the 52 countries that are visa exempt need to apply for an eTA prior to traveling to Canada.
An approved eTA is mandatory for all passengers including children. If you are entering Canada via automobile through one of the designated land borders with the United States then you are not required to apply for an eTA.
Citizens of countries that are not visa exempt must apply for a regular visa at a Canadian embassy or consulate.