Alright, here we go. Go to "run" and type
"%appdata%". After that, click on the ".minecraft" folder. Look in there for a folder named "saves". Open that up and leave that there. Next, go to "Minecraft.net", press download, and then click on and download "Minecraft Server.exe", which should be 270 kilobytes big. Alternatively, you could use "Minecraft Server.jar", which is smaller. Once you are done with that, copy the world that you want to convert (from the "saves" folder) and paste it in the server folder. Now delete the folder called "world", and then rename your map folder's name to "world". Then go to "Mincraft_Server". It will take some time to load, but will get there.