To apply for U.S. citizenship, you should meet the following basic requirements:
You should be at least 18 years old.
You should be a green card holder (permanent resident). For most people, the requirement is to have been a Permanent Resident Card holder for at least four years and nine months.
However, if you are married to a U.S. citizen, you should have been a green card holder for at least two years and nine months, during which time you have been married to your U.S. citizen spouse and continue to live in a conjugal relationship with him or her.