An Immigration Attorney for Southern California writes:
Although there are numerous paths to US citizenship or permanent residency, most begin with an employer or family sponsor. Here is an overview of the most popular types of family based petitions;
A-K-1 Fiance visa allows a U.S. citizen to sponsor a foreign national for 90 days in order to validate a marriage or leave the country.
A K-3 Marriage Visa allows a foreign national married to a U.S. citizen to obtain a visa.