Post offices are responsible for providing application forms for U.S. passports, and for sending the application to the U.S. State Department. The forms and a list of requirements can be obtained from most post offices. (Note, Passport applications are not accepted at all post offices.) You have to go in person to the post office to have the application processed and sent to the State Department. Allow at least 6 weeks for the application to be processed – sometimes more.
If you wish to travel outside of the Unite States, you must first have a passport. Passports also serve as a form of identification if you are trying to get a driver’s license, visa, or any other legal document.
If you do not have one, you may go to this website and get started: http://travel.state.gov/content/passports/en/passports/apply.html
If you already have a passport, and just need to renew it, you may go here: http://travel.state.gov/content/passports/en/passports/renew.html
There are fees that apply when renewing, or getting a new passport: http://travel.state.gov/content/passports/en/passports/information/fees.html
If you have a passport, but cannot seem to find it, or somebody has stolen it, you may go here to get started on ordering a replacement: http://travel.state.gov/content/passports/en/passports/lost-stolen.html