The N-400 form, or Application for Naturalization, is the application for naturalization. Only if you are a permanent resident (green card holder) may you apply and if you meet the following general eligibility: 

1. You are at least 18 years of age at the time of filing. 
2. You have been a permanent resident of the United States for the past 5 years- 3 years if married to and living with a U.S citizen for the past 3 years. 
3. You can demonstrate physical presence within the United States for 2.5 years (1.5 years if based on marriage to a U.S. citizen). 
4. You demonstrate good moral character. 
5. You demonstrate a basic knowledge of U.S. history and government (also known as “civics test”) as well as an ability to read, write, speak, and understand basic English; and 
6. You take an Oath of Allegiance to the United States 

The Naturalization application appears simple. However, this is the last chance for USCIS to screen the applicant. USCIS scrutinizes the manner you received your green card and reviews all applications previously filed.  USCIS looks for any misrepresentations previously made as well. If you have any concerns, it is best to consult an experienced attorney before filing. If you are confident, you may submit this application online.