Get Married Legally
You will need a legal marriage certificate to commence your application. This is the minimum legal requirement to prove that you are actually married to your American spouse.
Be aware that even if you do have a marriage certificate, U.S Immigrations Officials will still attempt to prove whether your marriage is genuine or merely a pretext to enable you to acquire U.S citizenship, thus you should make sure you have all the necessary documents and proof that can make your case stronger.
Apply for a Green Card
Once you have gathered sufficient proof of your marriage being genuine, your spouse will need to apply for a green card for you if you do not already have one. The issuance of a green card, also known as a permanent resident card, will permit you to live in the U.S and is itself a requisite for naturalization.
If you are already legally present in the US then you will need to apply for an “adjustment of status”. However, if you do not have a valid US visa and are not resident in the US then your application will be made by “consular processing”.
Essentially both processes involve a joint application by you and your spouse to have a green card issued to you. This process may take months to complete.
Fulfill Conditions for Naturalization
Should you have your green card approved, your next step will be to take up permanent residence in the US for a minimum period of three years after which you may apply for citizenship on the basis of marriage.
You will need to ensure that you meet the following criteria (extracted from the USCIS website):
Be 18 years of age or older
Be a permanent resident (green card holder) for at least 3 years immediately preceding the date of filing Form N-400, Application for Naturalization
Have been living in marital union with the U.S. citizen spouse, who has been a U.S. citizen during all of such period, during the 3 years immediately preceding the date of filing the application and up until examination on the application
Have lived within the state, or USCIS district with jurisdiction over the applicant’s place of residence, for at least 3 months prior to the date of filing the application
Have continuous residence in the United States as a lawful permanent resident for at least 3 years immediately preceding the date of filing the application
Reside continuously within the United States from the date of application for naturalization until the time of naturalization
Be physically present in the United States for at least 18 months out of the 3 years immediately preceding the date of filing the application
Be able to read, write, and speak English and have knowledge and an understanding of U.S. history and government (also known as civics)
Be a person of good moral character, attached to the principles of the Constitution of the United States, and well disposed to the good order and happiness of the United States during all relevant periods under the law
Apply for Citizenship
Having fulfilled the aforementioned requirements it is now time for you to take the bold step of applying for naturalization. You and your spouse will need to fill out application forms which are available online at the USCIS website (http://www.uscis.gov/us-citizenship).
Prepare for your Interviews and Citizenship Test
Once you have applied for citizenship, part of your naturalization process will involve be proving that you have a good command of English and of the government and history of the U.S as mentioned above.
You will need to prepare for the test by learning about the history of the USA, including the revolution, the formation of states, the presidents, the constitution and information about the more contemporary events regarding the United States of America.
Make the pledge
Finally your last step will be making the pledge of allegiance to the United States of America. At this stage, note that if you miss your appointment to take the oath of allegiance more than once, without good reason, your entire application may be cancelled.
The steps outlined here cover the general process of becoming a US citizen through marriage. However, the entire process is complicated and cannot possibly be conclusively covered in this guide.
Visit the Website of the United States Customs and Immigration, at this link-http://www.uscis.gov/us-citizenship, for more information or hire an immigration lawyer for further advice and guidance.