True..but does he send his letter of invitation in the mail to me so I can take to embassy in USA here or does he send it to the embassy in nigeria

I dont have my passport or visa yet but will be applying soon..

I am a U.S. citizen who has a boyfriend in nigeria. I am trying to plan for a trip to nigeria soon..it will be my first time visiting. I was trying to figure out should I have him write the letter of invitation, have it notarized and sent to me, or does he send it to the embassy? Also, has it been hard to get a visitor visa for anyone? One last question, ok, so we want to marry maybe soon or in the future, how long did any of u wait before marrying, was it your first visit or did you go back and visit again before marrying? I love to hear ideas, and information please..