If you have been in the United States and have had a green card for a minimum of five years, have put in a citizenship application and have taken the test, and your form N-400, Application for Naturalization, has been approved by USCIS, the final step is to get your naturalization ceremony scheduled.
There are actually two kinds of ceremonies that honor the taking of the Oath of Allegiance to the United States, an administrative ceremony and a judicial ceremony. USCIS administers the oath in an administrative ceremony, while in a judicial ceremony you are taken through the oath by a court. Those who take part in the administrative version will be given a package that welcomes you to US citizenship and takes you through the whole day. The package will be sent to you in the mail.
Something else that will be sent to you is a notice that provides you with the time and date of the ceremony. If you are unable to attend at that time, you need to return the Notice of Naturalization Oath Ceremony, Form N-445, to your local USCIS office together with a letter asking for a new date and the reason why you are unable to go to the scheduled ceremony.
When you get to the ceremony, you need to check in with an officer of USCIS and hand over the N-445 questionnaire, followed by your green card ID, in return for which you will receive the naturalization certificate, after which you officially seal your citizenship by taking the Oath of Allegiance.