Three ways you can help your fiance(e) or your spouse become a permanent resident
If you marry overseas, you can file a petition for your spouse with a USCIS office in the U.S. (or with a U.S. Consulate or overseas USCIS office in certain countries) after the marriage. Once that petition is approved, your spouse will be processed for an immigrant visa by the US Embassy or Consulate, and then the spouse could enter the US as a permanent resident.
If your fiance(e) is already in the U.S. with a lawful status, you can get married in the U.S. After the marriage, you can petition for the spouse so that the spouse can adjust status to a permanent resident and stay.
A third option is to bring your fiance(e) into the U.S. on a fiance(e) visa. You can file a petition with the USCIS first, so that the Consulate can invite your fiancee to apply for the fiance(e) visa. With that visa, your fiance(e) can enter the U.S. to have the marriage ceremony within the 90 days. After the marriage, your spouse can apply for permanent residence and stay.