US citizens should have a passport when crossing the border into Canada. Passports take about six weeks to receive once you apply, so apply early. If you need your passport faster than that, you can expedite it, but it will cost more money. You can apply for a U.S. passport at the U.S. Department of State website.
Children under the age of 15 who enter Canada by land or sea do not need a passport, as long as they are with their parents or legal guardians and present a certified copy of their birth certificate. Children arriving by airplane need a passport or passport card issued by the US government.
Visitors to Canada from the US do not need a travel visa as long as they plan to stay under six months. If you are traveling to Canada from the US and plan to stay longer than six months, you will need a travel visa. You can apply for the visa at the Citizenship and Immigration Canada website. You will also need to prove that you have enough money for your trip and that you have ties to the US to bring you back at the end of your visit. You might also need to pass a medical exam to ensure you are not bringing any disease into the country. A criminal background check may also be necessary.
US citizens may enter Canada without a passport, but must show proof of US citizenship when entering Canada. To prove this, bring a valid state issued ID card, such as a driver's license, as well as your certified birth certificate if you were born in the US, or your naturalization certificate if you were born in another country. However, keep in mind that when you re-enter the US, you must show a valid US passport if you enter by air, If you do not have a valid US passport, you can still enter the country legally by land or sea, but you will need a passport card, NEXUS card, or Enhanced Driver's License. In other words, it is easier to enter Canada than to re-enter the US. To avoid any hassle, get a valid passport before your trip.