Legal Permanent Residents (Green Card holders) can become US citizens through a process called naturalization.
Generally, anybody wishing to become a US citizen must meet the following requirements:
1) Age: He or she should be at least 18 years old to apply in his or her own behalf;
2) Presence: He or she must be physically present in the US at the time of the application;
3) Permanent Resident: Only those who have been legally admitted to the US as Permanent Residents are eligible;
4) Time Requirement: The applicant must have continuously resided in the US for at least five years and must have been physically present in the US for at least half of that period (30 months). Applicants must reside continuously in the state or the US Citizenship and Immigration Services (USCIS) district three months immediately before the filing of the application. The applicant must reside continuously in the US after the filing and until admission to citizenship;
5) Good Moral Character: An applicant must demonstrate, during all the five years before filing an application and until being sworn in as citizen, that he or she has maintained good moral character, believes in the principles of the US Constitution, and will not disturb the good order of the United States;
6) Basic English Literacy and Knowledge of US History and Government: Basic ability to speak, read and write English is a requirement. Applicants must also possess basic knowledge of US history and US government.
1) Filing. The applicant must file the following items with the USCIS center having jurisdiction over his or her area of residence:
a. Application for naturalization, Form N-400
b. Two color photographs (2 x 2)
c. Check or money order made payable to the United States Citizenship and Immigration Services, for the application fee
d. Copy of both sides of the applicant's Permanent Resident Card (Green Card).
2) Interview and Test. The applicant will be asked to appear before a naturalization examiner for interview and examination, after the above documents have been processed. The examiner will ask the applicant questions related to the personal information the latter submitted on his or her application.
The applicant will also take a test on US history and government, based on questions derived from the Federal textbooks on citizenship. He or she must also demonstrate basic ability to speak, read and write in English.
An applicant may prove his or her knowledge of US history and government and proficiency in English prior to the interview by passing a standardized test approved by the US Citizenship and Immigration Services.
3) Oath Ceremony. Upon approval of his or her application, the applicant is required to appear at a swearing-in ceremony. After which, a certificate of naturalization is either given to the applicant after the ceremony, or mailed to the applicant at a later date.