This is a simple guide to create an empty world - which is useful for many servers, as lobbies or maps. So, this is what you need to do:

1: Once you're logged in, go to singleplayer and "Create new world".

2: Configure it as you want, then change World Type to superflat and click Customize.

3: Now, click presets and type "0,0" in the text box.

4: Now, go back and click "Create new world".

that's it! Also, you should type "/setblock ~ ~ ~ stone" or else you won't be able to build there (if you don't got a mod/plugin for it of cource).