If you and your intended (who lives outside the U.S.) have not yet married -- or have held an informal ceremony that does not count as an official marriage in the location where it was held -- you can apply for a temporary (90-day) visa with which your fiancé can enter the U.S. and hold the wedding.
The U.S. citizen starts this process by filing a visa petition on Form I-129F with U.S. Citizenship and Immigration Services (USCIS). After USCIS approves the I-129F, it will transfer the case to the U.S. consulate in Port au Prince, Haiti. Your fiancé will apply for a K-1 visa through the consulate. This involves submitting forms and documents and attending an interview with a consular official. You, the petitioner, are allowed to attend this interview, though it is not required.
After your marriage in the U.S., your new spouse can apply to USCIS for a green card, through a process called adjustment of status. The two of you will attend a green card interview at a local USCIS office.