Well, basically, you need a letter of invitation, either private or business. I don't know how it goes with corporate letters, but if it is a private one, your friend has to go to the nearest court and fill a form, pay a fee, and get a letter which he/she can send. The issue is, inviting party (your friend) has a lot of legal and financial responsibilities related to your stay in Serbia and therefore don't expect anyone to do it for you unless they know you personally and very well. On top of that, inviting party is always being investigated by the police to make sure the invitation is a genuine one.
Another thing is, there is no direct flights from Nigeria to Serbia. Your best option is with Turkish Airlines. if you travel with any other airline (for instance Lufthansa, from Lagos or Abuja) you first need to get a valid transit visa.
Mind you, having a letter of invitation is not a guarantee whatsoever that the visa will be issued.