To work legally in Korea, you must obtain the appropriate employment visa. If you wish to work in Korea, you must obtain the visa outside the country. However, you can enter Korea on a tourist visa, obtain letters of sponsorship and apply for the visa in a nearby country (Japan or Hong Kong). As visa regulations and the documents required change quite frequently, you should contact the nearest Korean diplomatic or consular mission if you are outside Korea, or a Korean immigration office within Korea, for confirmation of regulations and fees.
On arrival in Korea, you must register at a Korean immigration office and obtain a residence permit and a re-entry permit within 90 days of entry. All foreigners are required to possess a valid re-entry permit in order to re-enter Korea. Korean immigration offices require the same documentation as is needed for obtaining the visa, so you should make plenty of copies.
Most English instructors are granted an E-1 visa (professor at an educational institution higher than a junior college), an E-2 visa (conversation instructor) or an E-5 visa (professional employee with a public relations firm or corporation). Dependants of diplomats stationed in Seoul can work as English teachers by obtaining a work permit from the Ministry of Foreign Affairs.
Despite what some employers may tell you, you are not required to hand over your Canadian passport to your employer for the duration of your stay. It is your passport and you should keep it in your possession.