How to make an Earth-like map for a server?

Hello everyone, I am looking for some help regarding world settings. I want to generate a map which works like SMP Earth where the map is spherical, i.e., the coordinates system of the game works like real-life coordinates system. I don't want to use the Earth map itself but I want to generate my own world. How do I make one and get it working on a server? Thanks in advance.