US citizenship is automatically acquired if born in the US or in any of its territories. Birthright citizenship is one of two paths to US citizenship, Naturalization being the other. You are also considered a US citizen by birth (in most cases) if you were born outside the US but both parents are US citizens, and at least one of them has lived in the US at some point in his/her life. Additionally, if at least one of your parents was a US citizen when you were born and your citizen parent lived in the United States for at least 5 years before you were born; and at least 2 of these 5 years in the United States were after your citizen parent's 14th birthday, you will be considered a US Citizen.
The process of an immigrant becoming a US citizen (not by birth) is termed as Naturalization. If you are not born in the US, you have to naturalize to become a US citizen. After the immigrant has lived in the US as a US permanent resident (green card holder) for a minimum of five years, he is eligible to apply for US Citizenship.
In order to be eligible for US citizenship, every eligible green card holder has to meet certain requirements unless they are citizens by birth or are applying as a US military personnel:
For a detailed description of these US citizenship requirements it is important to have a checklist and cross-out each requirement as you meet it.
Besides checking for eligibility, a potential US citizen has to fulfill a step-by-step process to acquire US citizenship. It starts out by filling Form N-400 and submitting it with photographs. Once you are called in for collecting your biometrics, you will be scheduled for an interview and test to evaluate your US civics knowledge and language proficiency. Once all these formalities are completed, you await a notification regarding date and time you can take the oath of allegiance to the US.
The US Citizenship interview and civics test is a crucial stage of the US citizenship process. One has to prepare well and be ready for both. Once your biometrics have been collected, you should expect a mail notifying you of the date and time of your interview.
At the interview you will be asked questions regarding your application, your history and current presence in the US. Answer truthfully and to the point. You will also be tested on your knowledge of US history and US civics. This could be either verbal or written or both. You can access sample questions fro the test here.
This is the final stage of acquiring your US citizenship journey. Once you take your oath, you are officially a US citizen.
Once your interview is completed and you have successfully passed your civics test, you are sent Form N-445, Notice for Naturalization Oath Ceremony. This will have the date and time of where you are to take your oath. If you are unable to make it, you have the provision to opt for another date with the same form. Once you take your US citizenship Oath and promise to uphold the US constitution, you are a US citizen.
For those who have applied for a US Citizenship - Everything you Need to Know or are planning to apply for a US Citizenship - Everything you Need to Know, here is some useful information on the process.