first you need
winrar
mediafire account
ok get all that
go to %appdata%
and then go to .minecraft
and then go to saves
get the world you want and copy
NOT cut
and copy it to desktop
and then right click on it and click add to archive
and then click ok
then a new folder should come up on your desktop

then open up media fire and go to my files and drag the new folder on the desktop in to media fire
and then click on the file on media fire and then the new window that comes up copy the URL
and copy that in to the box on planet minecraft