U.S. citizenship can only be obtained by birth or through naturalization. Naturalization is the process of obtaining U.S. citizenship for green card holders. Once a green card holder obtains citizenship, he or she must surrender the green card.
Becoming a U.S. citizen has many advantages including the right to vote and the ability to sponsor relatives to come to the United States.
For U.S. citizenship, you must meet the following requirements:
1) Be at least 18 years old
2) Have had a valid Green Card at least five years. If you are married to U.S. citizen you may apply after three years with a valid Green Card. Or, if you have served in the United States armed forces during war, you may apply for U.S. citizenship without first obtaining a Green Card if you were in the United States upon enlistment into the U.S. military.
3) Have maintained continuous residence in the United States for at least five years (or three years if you are married to a U.S. citizen)
4) Have been physically present in the United States for at least two and a half years (or one and half years if you are married to a U.S. citizen
5) Have lived in the state where the Form N-400 is submitted for at least three months
6) Be able to read, write and speak English
7) Have a general knowledge of the fundamentals of U.S. history and government
8) Be a person of good moral character and willing to abide by the principles of the U.S. Constitution.