Assuming that he or she entered the U.S. legally using a visa that allows adjustment of status like an ESTA Visa Waiver or B-2 tourist visa, then after you marry, you will need to file an adjustment package. This includes the following forms: I-130 (Petition for family member), I-485 (application for Lawful Permanent Residency), I-765 (Application for work permit), I-131 (Application for Advance Parole), G-325a (Biographic data - one for each of you), I-693 (Medical exam that a certified doctor must fill out), and I-864 (Affidavit of support). You will need to file each form with supporting evidence and appropriate filing fees. You can find these forms at www.uscis.gov/forms.
The supporting evidence that you would file would be birth certificates, marriage certificate, divorce certificates if either of you have been married previously, proof of your U.S. Citizenship, proof of your spouse's legal entry into the U.S., and financial documents to prove your income over the last year at least.
In about 3 to 5 months after filing, the work permit and Advance Parole (travel permit) should be issued. About 5 to 7 months after filing the interview should be scheduled (though now it is taking longer, so maybe closer to 7 to 9 months). If all goes well, a few weeks later the U.S. Lawful Permanent Residency card (green card) should come in the mail.
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