Been talking to my boyfriend from Nigeria for months now. How do I get him to the USA for a visit?

If you want him to just visit, he needs to apply at the US consulate in Nigeria for a B visa. If you have physically met within the last two years you can sponsor him for his K visa and get married when he enters the US within 90 days. Third you can fly to Nigeria and marry him there and then he can apply for his spousal K3 visa. Finally, if you don't want to fly to Nigeria, he can enter on a visitor visa and then get married return to Nigeria and process at the US consulate there for his green card. I would not recommend him coming here to visit, and adjusting status after marriage. That would technically constitute immigration fraud. Counsel  anywhere in the USA can represent you and some of us charge a very affordable flat fee for representation throughout the life of the case.