Citizens of following countries do not need a visa to visit Canada: Andorra, Antigua and Barbuda, Australia, Austria, Bahamas, Barbados, Belgium, Botswana, Brunei, Cyprus, Denmark, Estonia, Finland, France, Germany, Greece, Hong Kong, Iceland, Ireland, Israel, Italy, Japan, South Korea, Liechtenstein, Luxembourg, Malta, Mexico, Monaco, Namibia, Netherlands, New Zealand, Norway, Papua New Guinea, Portugal, Samoa, San Marino, Singapore, Slovenia, Solomon Islands, Spain, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Swaziland, Sweden, Switzerland, United Kingdom, United States.
From the United States
Keep your visa documents when leaving the United States of America
If you are not a US citizen holding a visa for the US (including the green "waiver" visas people from Western countries get at US borders), you will have both a visa stamp in your passport and a loose immigration document (I-94 or I-94W card) that the US customs officer puts in your passport. When entering Canada from the US (either by land or by plane): if you intend to come back to the US after your stay, do not try to hand the I-94 or I-94W immigration document back to the border officers (they normally don't ask for it). You can enter the US multiple times during the time allocated to your visa (for Western tourists, normally 90 days), but you need to have the immigration document as well to validate the visa. If you come back from the US without that document, you will not only have to apply again for a new visa, but you will also be asked severe questions by US immigration. So keep the immigration document with you until you leave North America for the last time in your trip.
You are likely to arrive to Canada by air, most likely into Montreal, Toronto or Vancouver (the 3 largest cities, from East to West). But other airports in Canada also have international (mostly from the US) flights as well, particularly (from east to west), Halifax, Gander, Moncton, Ottawa, Winnipeg, Regina, Saskatoon, Calgary, Edmonton, Cranbrook, Kelowna and Victoria.
Air Canada is the country's only national air carrier, covering the entire country and international destinations. WestJet is a carrier based in Western Canada that is continually expanding their service eastwards. There are a few discount domestic companies, which offer flights to all major cities, with connections to smaller ones. As with most airlines, it's cheaper if you book your flight ahead of time, but bookings can be made right up to the last minute if you've money to spare. You can find these airlines easily online.
Although less likely, you might also enter the country by road from the United States through one of the (literally) hundreds of border crossing points. Obviously, the same rules will apply here, but if your case is not straightforward, expect to be delayed, as the officials here (especially in more rural areas) see fewer international travellers than at the airports. Also expect delays during holiday periods, as border crossings can become clogged with traffic.
Drivers of American cars will need a certificate confirming that they carry enough public liability insurance (generally $200,000) to meet the requirements of all Canadian provinces and territories. Since many US states permit limits below this threshold, American visitors bringing their own automobiles should check with their automobile insurers and obtain the required certificate.
When driving within Montreal or Toronto keep in mind that these cities are densely populated and parking can be difficult to find and/or expensive. Both cities provide extensive public transit, so it is easy to park in a central location, or at your hotel or lodging, and still travel throughout the metropolitan areas.
Via Rail is Canada's national passenger rail service. Amtrak provides connecting rail service to Toronto from New York via. Niagara Falls, Montreal from New York and Vancouver from Seattle via. Bellingham. The train is a very inexpensive way to get into Canada, with tickets starting from as low as $43 (U.S.) return to Vancouver. There is also thruway service between Seattle and Vancouver.
Be wary though. Not many private citizens in Canada take the train as a regular means of transportation. Most citizens simply drive to where they want to go if the distance is short (which in Canada can still mean hundreds of kilometres!), or fly if the distance is long.
Greyhound Canada serves many destinations in Canada, with connecting service to regional lines and U.S. Greyhound coaches. Be sure to inquire about dicounts and travel packages that allow for frequent stops as you travel across Canada.
In British Columbia, you can enter Canada by ferry from Alaska and Washington. Alaska Marine Highway serves Prince Rupert, whereas Washington State Ferries serves Sidney (near Victoria) through the San Juan islands. There is a car ferry from Victoria to Port Angeles run by Black Ball; there are also tourist-oriented passenger-only ferries running from Victoria to points in Washington.
There is a car ferry from Nova Scotia to Maine run by Bay Ferries (Yarmouth-Bar Harbor).
There is a passenger ferry running from Fortune in Newfoundland to Saint Pierre and Miquelon.
A small car ferry operates between Wolfe Island, Ontario (near Kingston) and Cape Vincent, NY.
The CAT car ferry between Rochester, NY and Toronto, Ontario was discontinued in January 2006.