Generally, to be eligible for naturalization you must:

• Be age 18 or older; and
• Be a permanent resident for a certain amount of time (usually 5 years or 3 years, depending on how you obtained status); and
• Be a person of good moral character; and
• Have a basic knowledge of U.S. government (this, too, can be excepted due to permanent physical or mental impairment); and
• Have a period of continuous residence and physical presence in the United States; and
• Be able to read, write, and speak basic English.

There are exceptions to this rule for someone who at the time of filing: 

• Is 55 years old and has been a permanent resident for at least 15 year OR
• Is 50 years old and has been a permanent resident for at least 20 years OR 
• Has a permanent physical or mental impairment that makes the individual unable to fulfill these requirements. (Source: USCIS)

You may be able to apply for naturalization if you are at least 18 years of age and have been a permanent resident of the United States:

• For at least 5 years OR
• For at least 3 years during which time you have been, and continue to be, married to and living in a marriage relationship with your U.S. citizen husband or wife OR
• Have honorable service in the U.S. military.

Certain spouses of U.S. citizens and/or members of the military may be able to file for naturalization sooner than noted above.

Every applicant for naturalization is scheduled for a naturalization interview with USCIS in which they must demonstrate an ability to read, write and understand English plus pass a verbal test on US history and civics. 

The final step in becoming a naturalized U.S. Citizen is an Oath Ceremony scheduled by the USCIS. At that ceremony, each applicant will take the Oath of Allegiance for U.S. Citizenship.