First you have to get married and get legal.
The best and most recommended way is for the U.S. citizen to file for a fiance(e) visa for the immigrant first. If you go directly to the USCIS, tell them that your fiance(e) is still in his/her home country and not already in the U.S. illegally. Then when he/she enters with his/her fiance(e) visa, you, the U.S. citizen have 90 days to get married. After getting married the now immigrant husband/wife can file for Permanent Resident Alien Status ("green card"). After about 3 to five years after that he/she can apply for U.S. citizenship if he/she wishes.