Hey nairalanders, compliment of the season to you all. So it happened that towards the last quarter of this year, I met a guy on a dating site, he is actually a black American based in New York, since then it has been a smooth ride, this guy is pretty serious with me, he can call me 20 times in a day, atimes I get tired and ignore his call, but one thing I like about him is his attitude, very homely and humble. I've always had this impression that black Americans are unruly and bunch of scallywags, but he is different, this December, he told me he wanted to visit me in Nigeria, was talking about marriage and all, but I discouraged him from coming, since I've got no idea of how this stuff works , and I felt like we were moving too fast, so I don't look desperate, so I recently told my elder brother about him, he told me not to waste time and go for it. So I plan on him coming to Nigeria before my birthday, which is by April, how do we go about the visa and all, I've never done this before, please I need you all guidance and input, should I still delay his coming, and If things work out eventually and we tie the knot, how do I file to join him, he is serious and I'm the one trying to slow things down.

Let him come. He can stay in your house for some days till you are comfortable enough with him, then you guys can rent airbnb or you shuttle between the two.
Visa is straightforward if he has all documents. See Nigeria immigration service website for the documents he needs, depending on the type of visa.
He will apply for a visa and when its granted, he flys to Nigeria and you wait at the airport.

Get to know him first, then you talk about getting married. I think there is a thread for USA spousal visa here. You have to search. That country wahala plenty abeg