A completed Form N400 (US Citizenship Application) has to be filed along with photos and supporting documents. A green card holder who is above 18 years of age and meets other eligibility requirements can file for Citizenship using N400 form. Becoming a U.S. citizen has many advantages including the right to vote and the ability to sponsor relatives to come to the United States.

OTHER IMMIGRATION SERVICES AND USCIS FORMS/USIS
how can I get my green card
I-821D
how to renew green card
i 539
travel.state.gov
travel document