US citizens who get married to foreign nationals and who wish to bring their spouses to America, must sponsor their spouses for lawful status in the United States and the spouses or fiance(e)s of US citizens must obtain visas to enter into America. K non-immigrant visas are meant for the spouses and fiancé(e)s of US citizens and they must obtain these visas to enter into the United States.
However, there are few different ways to sponsor your fiancé(e) or your spouse and if your fiancé(e) lives in a foreign country, you must first file Form I-129F, Petition for Alien Fiancé(e), for her. After the approval of your petition, your fiancé(e) must visit a US Embassy or Consulate in her home country and file an application for a K-1 visa. Once she enters into the United States with that visa, you must get married to her within 90 days from the date of her arrival. That is because, she can file an application for adjustment of status only after getting married.
You need to remember that people who enter into America with fiancé(e) visas must get married to their US citizen fiancé(e)s as soon as they get into the United States. Similarly, fiancé(e) visas cannot be extended and the person who enters into the country with a K-1 visa must get married to the sponsoring US citizen within 90 days, if not, the K-1 visa holder must leave the country at the end of the 90 day period. Moreover, K visa holders cannot change their status and they can only apply for adjustment of status and become Green Card holders after getting married.
If your fiancé(e) enters into the country with some other non-immigrant visa, you will have to file Form I-130, Petition for Alien Relative, for her, after marrying her. In such cases, your wife may file Form I-485, Application to Register Permanent Residence or Adjust Status and become a lawful permanent resident and she can file this form while you file Form I-130, for her.
If you marry a person who is abroad and if you seek to bring your spouse to the United States, you must first file Form I-130 for your spouse and your spouse who is currently in a foreign country, must file an application for a K-3 visa. She may then get into the United States as a permanent resident after the approval her application.