If you plan to enter Canada by ground you can apply for a U.S. Passport Card. The card is less expensive than a Passport book and allows you to cross the border at land crossings or sea-ports-of entry. The card cannot be used if you are traveling by air.
If you are flying into Canada you'll need a U.S. Passport book. All the forms you need are available on the Department of State website or at the Passport Office located in your local post office. Both locations have information specific to Canadian travel and border requirements.
You must apply for a passport book in person at a passport office. You'll need to fill out a DS-11 application and you must submit proof you are a U.S. citizen like a birth, naturalization or citizenship certificate. Don't forget two passport pictures. The cost is about $110. The cost for a passport card is about $30.Forms are available either online at the Department of State website or you can pick one up at your local Passport Office located inside the Post Office.