This one's not so easy--the Department of State website reports:
ENTRY/EXIT REQUIREMENTS: All visitors to Nigeria require a passport and a Nigerian visa to enter the country, and must obtain the visa in advance from a Nigerian embassy or consulate. Such visas cannot be obtained on arrival at the airport. Promises of entry into Nigeria without a visa are strong indicators of fraudulent commercial schemes in which the perpetrators seek to exploit the foreign traveler's illegal presence in Nigeria through threats of extortion or bodily harm. Moreover, you cannot legally depart Nigeria unless you can prove, by presenting your entry visa, that you entered Nigeria legally. Visit the Embassy of Nigeria web site for the most current visa information.
On the Nigeria visa application FAQ page, if you want to get your visa quick, it costs $85.00, and they want you to provide the following:
Tourist / Visitor Visa:
Passport valid for at least 6 months.
Duly completed visa application form
Two recent passport photographs
A Letter of invitation from a company/host in Nigeria accepting immigration responsibility.
Visitors/tourists are required to show evidence of sufficient funds
Evidence of online payment for visa fee
https://portal.immigration.gov.ng/?p=visaguidelines