Some questions from a complete newbie

[Geekbot_376]

Hi,

I'm new to MCServer, and I'm hosting a server for me and my brother, but I want to know how to:

use a world save from minecraft as the world
make the generated world a superflat
enable command blocks
switch gamemode and allow/disallow commands/cheats (/give mostly)
Install plugins

Any answers to any of those questions would be appreciated, and would stop me being like this about the whole thing