In order to become a United States Citizen, you must be (1) born in the United States, (2) be born to US Citizen parents, or be a Legal Permanent Resident (Green Card holder) and apply to become a naturalized United States (US) Citizen.
Generally, in order to become a citizen of the United States, a person must meet the following criteria and must make an application for naturalization (Form N-400, usually1):
1. Must be a Lawful Permanent Resident (Green Card holder) for at least 5 years (3 years, if married to a US Citizen)
2. Must be 18 years or older
3. Must have lived in the US continuously for 5 years (3 years, if married to a US Citizen)
4. Must be a person of Good Moral Character (GMC)
5. Must be able to speak and write in English or have a medical waiver
a. Exception for Green Card holders over 55 years old who have had their green card for 15 years or longer –or- over 50 years old who have had their green card for 20 years or longer
6. Must pass History and Government Test
Each of these requirements have sub-requirements and some exceptions. If you have had any problems with the police, owe child support, failed to register with the selective service, failed to do your taxes, separated from US Citizen spouse, or left the United States for long amounts of time or left the United States often, you should contact an Immigration attorney before filing your application for Naturalization.
1There are certain situations where one does not need to file Form N-400 such as certain members of the military and children of Green Card holders that became US Citizens while the child was a minor. To determine which application would be appropriate, contact an Immigration attorney.