If you are a foreign-born person who has married a U.S. citizen, and you are currently staying or living in the U.S., your marriage may qualify you for a green card (U.S. lawful permanent residence), and to apply for it through a process known as adjustment of status (AOS).
First, however, you need to make sure that you are eligible for a marriage-based green card, and eligible to apply for it by adjusting status. If so, we'll give you some helpful information on how to prepare your marriage-based application.
Marriage to a U.S. citizen makes you what is known as an "immediate relative," and eligible for a U.S. green card just as soon as you can get through the application process. (Contrast this to the situation faced by "preference relatives," who face numerical limits on the green cards given out annually, and therefore waits of many years after their spouse begins the immigration process before they can move forward and apply for the green card.)
This assumes that your marriage is legally valid in the place you were married. For example, some U.S. states prohibit marriages between people who are minors, or those closely related by blood.
It also assumes that your marriage is "bona fide" -- that is, not just a sham to get a green card.
Good news here if you overstayed a U.S. visa: While most people in your shoes are not eligible to adjust status (and must instead apply for their visa through consular processing, in their home country), your marriage to a U.S. citizen basically cancels out the issue, and you can apply to adjust status no matter how long you were out of status. In fact, you'll be getting a double benefit by doing so, because if you instead chose to apply through consular processing, and your period of unlawful presence in the U.S. was 180 days or more, you'd be subject to the time bars on your return described in the article, "Three-Year and Ten-Year Time Bars for Unlawful U.S. Presence."
An important caution applies to the above, however. If you used a visa to come to the U.S. with the existing intention to apply for a marriage-based green card, you have committed visa fraud, and may be denied the green card on that basis. It's far better if you weren't yet married when you entered the U.S. and only met your spouse or decided to get married later.
The news on eligibility to adjust status isn't so good for people who entered the U.S. without inspection, for example by making their way across the border alone. They are not eligible to adjust status unless they happen to fall under an old section of the immigration law called 245(i), and will have to apply for their green card through consular processing and possibly face (or request a waiver of) the three- and ten-year bars described above. See an attorney if you are in this situation.
In order to adjust status, you must fill out Form I-485, along with supporting forms and documents. This form is issued by U.S. Citizenship and Immigration Services, or USCIS. Follow the instructions in the article, "What You Need to File I-485 for Adjustment of Status."
You'll see in that article that one of the things you must include with Form I-485 is proof that you are eligible for a green card. In your case, that means proving that you are married, and that your spouse wishes to petition for you. You may have already taken care of both things if your spouse already submitted Form I-130, Petition for Alien Relative, to USCIS (with a marriage certificate) and received an approval notice. But don't worry if you haven't already done this: You can submit Form I-130, with the marriage certificate and appropriate fee, to USCIS at the same time that you submit the rest of your adjustment of status paperwork.
Also note that, because this is a family-based immigration case, the U.S. citizen spouse will need to file the Affidavit of Support on Form I-864 for the immigrant. The purpose is to show that the U.S. spouse will support the immigrant financially for approximately ten years into the future, thus avoiding the immigrant having to rely on need-based public assistance (often called welfare).
The immigrant will also need to undergo a medical exam, and include the results with the AOS paperwork.
After your application is processed by USCIS and the immigrating spouse has attended a fingerprinting appointment, you will both have to attend an AOS interview. You can also hire an attorney to represent you during this interview.
USCIS will send instructions on what to bring to this interview. Be sure to bring a well-chosen selection of documents (copies and originals) proving that your marriage is the real thing and that you are sharing a life together. For example, joint bank statements, a mortgage or apartment lease in both your names, wedding photos, and children's birth certificates are all good forms of evidence.
The USCIS officer will ask about basic information in your application, and ask various questions about your marriage. See "What Happens at The Green Card Marriage Interview?" for sample questions and other tips on preparing.