In order to become a U.S. citizen, you must

(1) have been admitted to lawful permanent residence for five years (three years if Green Card obtained through marriage to U.S. citizen);

(2) you must be 18 years old;

(3) you must be maintain continuous residence for five years (three years if Green Card obtained through marriage to U.S. citizen);

(4) you must be physically present in the U.S. for at least half of the 5 years (or half of the 3 years if you obtained a Green Card through a U.S. citizen spouse);

(5) you must be a person of good moral character for the 5 years (or 3 years if the alien obtained a Green Card through a U.S. citizen spouse);

(6) you must demonstrate an elementary level of English (reading, writing, understanding); and,

(7) you must have knowledge and understanding of the fundamentals of history and government of the U.S.

Special exceptions to some of the general requirements are available for the disabled, members of the military, veterans, spouses married to U.S. citizens living overseas, and Legal Permanent Residents who work for certain organizations that promote U.S. interests abroad. Similarly exemptions from the English language requirements are available for those over 55 years who and have lived in the United States as a Legal Permanent Resident for 15 years, or are over 50 years old and have lived in the United States as a Legal Permanent Resident for 20 years.

Check out the INS website for more information.