If you have acquired a US Green Card, become a permanent resident of the United States, and have lived in the United States long enough as it required and meanwhile meet other eligibility US citizenship requirements, then you can fill in the immigration form N-400. In order to avoid unnecessary delays or rejections, it is especially important to fill in the N-400 completely and correctly. Here are some tips for properly filling out the application:
You must be at least 18 years of age or older.
Must be a legal permanent resident for 5 years (or if married to a US citizen, it can be 3 years).
Staying in a continuous residence for the past 5 years (3 years if you marry a US citizen).
Maintain physical presence for the past 5 years (3 years if you marry a US citizen).
Have a good moral quality.
Have a basic understanding of English.
Have a basic understanding of the US government and history.
Willing to swear allegiance.