A valid passport is required, and most visitors are required to have a visa. You should obtain your Nigerian visa from a Nigerian embassy or consulate in advance of your travel. In most cases, you cannot obtain a visa upon arrival at the airport.
You cannot legally depart Nigeria unless you can prove, by presenting your entry visa, that you entered Nigeria legally.
U.S.-Nigerian dual-national citizens are now required to have a valid Nigerian passport in order to depart the country. Dual-national citizens can be, and often are, denied boarding until they have obtained current Nigerian passports.
Some HIV/AIDS entry restrictions exist for visitors and foreign residents of Nigeria. Nigerian authorities may deny entry to foreigners who are “undesirable for medical reasons” and may require HIV tests for foreigners intending to marry Nigerian citizens. Please verify this information with the Embassy of Nigeria before travel.