How can I become a U.S. citizen?

If you are already in the USA or if you are married to an American, or wish to marry an American, then one must fill out further forms in order to obtain U.S. citizenship.

Regardless as to whether you are married to an American or not, without the approval of the appropriate application, you are not allowed to obtain work.

What you should pay special attention to is the following:

Worksheet

With regard to the following questions you must answer “true” or “false”:
I am at least 18 years old.
I am a Permanent Resident of the United States, and I have been issued a
Permanent Resident Card (formerly called Alien Registration Card).
I have been a Permanent Resident for:
5 years or more,
3 to 5 years,
less than 3 years.
During the last 5 years, I have NOT been out of the United States for 30 months or more.
Since becoming a Permanent Resident, I have not taken a trip out of the United States that lasted for 1 year or more.
I have resided in the district or state in which I am applying for citizenship for the last 3 months.
I can read, write, and speak basic English.
I can pass the civics test.
I am a person of good moral character .
One of the following is true:
I am female, OR
I am a male registered with the Selective Service, OR
I am a male who did not enter the United States under any status until after my 26th birthday, OR
I am a male who was born before January 1, 1960, OR
I am a male who was in the United States between the ages of 18 and 26 but who did not register with the Selective Service, and I will send a “Status Information Letter” from my Selective Service explaining why with my application.
I have never deserted from the U.S. Armed Forces.
I have never received an exemption or discharge from the U.S. Armed Forces on the grounds that I am an alien.
I am willing to perform either military OR civilian service for the United States if required by law. (Note: if your religious teachings and beliefs prohibit you from performing military service, you must be willing to perform non-military service)
I will support the U.S. Constitution.
I understand and am willing to take an oath of allegiance to the United States.

Requirements

Before a permanent resident can apply and obtain American citizenship, they must first have an understanding of American history, as well as the structure of the American government. Surprisingly the U.S. Citizenship and Immigration Service offers help with studying.
Application form: N-400 (available to fill out online)
Basic American History
Basic American Government structure
Naturalization Test Questions
Test Yourself on U.S. History
100 Sample U.S. History Questions with Answers
Oath of Allegiance
Where does one file the naturalization application in order to obtain U.S. citizenship?