To apply for naturalization you will need to have 30 months of physical presence in the U.S. ( or 18 months if you are married to and living with your U.S. citizen spouse) immediately proceeding the day your N-400 application is received by USCIS. A high number of days outside of the U.S. (around 900) will cause the examining officer to look very closely at the application to determine if the physical presence requirement is met.
Rules for Continuous Residence
5 years of Continuous Residence is required. Continuous Residence means place of general abode. Where do you lay your head at night? Where do your call yourself home? It's not enough to have a U.S. address and a bank account, if you live and work overseas. USCIS looks at factors such as employment, in the U.S., place where your immediate family lives, retention of full access to a U.S. home and not obtaining employment abroad. Any Absence of more than six months breaks the residence period and causes the 5 year clock to start counting all over from the time of the re-entry. If you live temporarily overseas it is important to seek an immigration attorney's advice to plan for your naturalization application so that you meet the requirement of continuous residence when the time comes to file. Entering the U.S. once every six months for a few weeks will not satisfy this requirement. There are avenues to preserve the residence in the U.S. while living overseas in certain cases
Good Moral Character
You must have five years of good moral character preceding the date that the N-400 application is filed with USCIS. If you have any type of criminal conviction, arrest or probation anywhere in the world they must be disclosed on the application. This is true even if you have been told that they will not appear on your record. It is suggested that if if you have to answer "yes" to any of the moral character questions on the N-400 application pertaining to criminal record, prior deportation, prior voting etc.. that you seek an immigration attorney's counsel in filing the N-400 application.
Knowledge of English Language and U.S. History
You must demonstrate a knowledge of written, spoken, and understanding of elementary English language and U.S. History. The officer will expect you to be able to answer their questions about the N-400 application. Also, you will receive a sentence to read and write at the interview. Finally, you will need to answer correctly 6 out of 10 questions on U.S. History taken from a list of 100 questions. There are exceptions to this for LPRs over the age of 50 and 55 who have help LPR status for 20 and 15 years. These LPRs do not have to read and write, but must answer the history questions in their own language. Also, there are exceptions for those LPRs over the age of 65 with 20 years as an LPR. For these LPRs the history questions are chosen from a group of only 10 questions. Finally, there are medical waivers for those who show that they cannot learn to read, write or answer questions of history. N-648 waivers must be done by a physician with medical evidence of the disability.
Must be 18 years for N-400
Those under the age of 18 may qualify to file N-600 through their U.S. citizen parent or parents with whom they are living. If you are close to the age of 18, then wait until you turn 18 to file the N-400. If you turn 18 while the N-600 is pending the application will be denied and you will have to re-file the N-400 losing the N-600 filing fees.
Must be attached to the principal of the U.S. Constitution and Willing to Bear Arms
If you need to answer "no" to any of the questions about loyalty to the U.S. or wiliness to bear arms or perform non-combat services for the U.S. if called on, you will need to seek immigration counsel in filing your N-400. There are exceptions to these rules but they are complex and will need to be considered carefully.
Additional resources provided by the author
A good source of information is the USCIS.gov website. This site provides practice N-400 tests, sample interviews, and forms for filing.