I have a boyfriend in Nigeria how do I bring him to the USA so that we can get married here in the USA

You can either pursue the route of a fiancée visa, or you can get married and pursue the route of a spousal visa. If your boyfriend travels to the United States on a visitor visa, with the intent only to visit, but later changes his mind and decides to get married and remain, he can adjust status, even though he falls out of status. ( as long as you're a US citizen) You should retain counsel here in the state of Florida for representation.