You must obtain an Electronic Travel Authorisation before to fly into Canada. It costs 7 CAD. Most nationalities, including American citizens, will need the eTA before flying into Canada.
However, if you drive to Canada or arrive by train, bus or boat, you don’t need to apply for the eTA! Just make sure to have your passport.
When arriving, immigration control officers will most likely ask you to show that you have enough funds to support yourself on your trip. They are also likely to ask you to show proof that you plan to leave Canada, and where you are planning on staying.
If you decide to couch surf, be sure to ask your host for their full name, number, and address.