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U.S. Citizenship Requirements

To be eligible for U.S. citizenship, you must:

Be 18 years of age or older
Be a legal permanent resident, also known as a green card holder, for at least five years, or three years if you are married to a U.S. citizen. You may submit an application for naturalization to the Immigration and Naturalization Service (INS) three months before you reach the residency requirement.
Be a person of good moral character. You must submit a set of fingerprints to the INS in order to be considered for citizenship. The INS then sends the prints to the FBI for a thorough background check.
Be able to speak, read, write and understand ordinary English words and phrases. Some exemptions are granted for this requirement.
Be able to demonstrate knowledge and understanding of the fundamentals of U.S. history and principles of government.

You must also:

Submit an application for naturalization to the INS with the proper fee (currently $225.00) and two photos.
Attend a preliminary interview conducted by an INS officer.
And if approved for citizenship, take the Oath of Allegiance to the U.S.

To Renounce your citizenship you must do it intentionally and sign a document in the presence of a US consular official.