Major international airports are spread across the country from Vancouver in the west to Halifax in the east. Many international visitors choose to arrive at U.S. border cities (e.g. Seattle, WA and Buffalo, NY) and then drive across the border. This can be a cheaper way of entering Canada, but be prepared for traffic and extended waits at border checkpoints during peak travel times.
Major cities like Toronto, Vancouver, Calgary, Montreal, and Halifax have excellent public transportation networks: taking the bus, riding the metro, or hailing a cab is a breeze. Smaller cities and rural towns are walkable, but you'll need a car to get out into the countryside. Public transportation in and out of national parks is virtually nonexistent. The space between metro centers can seem daunting, so consider flying if you wish to cut down on travel times.