A U.S. Citizen who wishes to marry a non U.S. Citizen or permanent resident can help their fiance obtain permanent residence in different ways.
One way is to apply for a fiance visa: If your fiance is overseas and you want to marry in the United States. This lets your fiance enter the United States for 90 days so that your marriage ceremony can take place in the United States. Once you are marry, your spouse can apply for permanent residence and you can remain in the United States while your application is processed.
When a U.S. citizen wishes to bring over a fiance, he or she will need to demonstrate that he/she has met their intended partner within the past 2 years, that both parties are free to marry, and that there are sufficient financial resources to support the future spouse.
After the I-129F is approved, the K-1 visa will be issued to the fiance and the K-2 visa can be issued to the K-1 Fiance. Then within 90 days of entering the U.S. the parties must marry and the K-1, K-2 applicants can apply for adjustment of status.