Your Nigerian Fiancee needs permission from the US government to allow her to enter the USA. This is called a K1 Fiancee visa.
Fiance visa applications are held only briefly at NVC, just long enough for NVC just long enough for NVC to assign a Lagos case number and forward your file via diplomatic pouch to Nigeria.
Once her petition arrives in Nigeria, your fiancee goes online to schedule her interview.
Two weeks before her interview date she undergoes a medical exam at one of the two Consulate approved clinics in Lagos
Finally she attends the visa interview at the consulate in Lagos, to demonstrate she is of good moral character and the engagement is Bona fide.
A week later a courier delivers her visa, and then she can join you in the USA.
By Fred Wahl
Your Personal Immigration Guide