BeforeYouGo.com

For US citizens, a US Passport is required when traveling outside the United States.  There is also a new Passport Card that can be used instead of a Passport for land and sea travel between the US and Canada, Mexico, the Caribbean, and Bermuda.  Of course, a normal Passport can also be used for those destinations so a Passport Card isn't really that useful, it's just a smaller, more limited version of a passport.

Fees:
Passport fees are currently:
Adults (age 16 and older):
New Passport and card:    $120
New Passport:    $100
New Card:    $45
Renew Passport and card:    $95
Renew Passport:    $75
Renew Card:    $20

Minors (under 16 years old):
New or renew Passport and card:    $95
New or renew Passport:    $85
New or renew Card:    $35

Where to go:
To get a new passport you'll want to go to an Acceptance facility, which are most often a post office.  You'll need to check before you go about how the facility you are going to accepts payment, some will take checks, cash, or credit cards, but some won't.

Photographs:
You will also need and 2 pictures and they are very picky about the size and quality of the pictures.  They need to be:

1. Identical color pictures
2. A size of 2 x 2 inches
3. Taken within the past 6 months, showing current appearance
4. Full face, front view with a plain white or off-white background
5. Between 1 inch and 1 3/8 inches from the bottom of the chin to the top of the head
6. Taken in normal street attire:
   •   If you normally wear prescription glasses, a hearing device, wig or similar articles, they should be worn for your picture
   •   Dark glasses or nonprescription glasses with tinted lenses are not acceptable unless you need them for medical reasons (a medical certificate may be required)

Some post offices will be able to take these photographs for you, but it is usually cheaper to have them taken on your own first.  Costco and Sams Club are two low price options.   

Documentation:
You will also need a couple items of documentation to prove you are who you say you are:

• Previously issued, undamaged U.S. Passport
• Certified birth certificate issued by the city, county or state
• check box Consular Report of Birth Abroad or Certification of Birth
• Naturalization Certificate
• Certificate of Citizenship

AND

• Previously issued, undamaged U.S. passport
• Naturalization Certificate
• Valid Driver's License
• Current Government ID (city, state or federal)
• Current Military ID (military and dependents)

You can find more information on passports and report lost or stolen passports here.