First you go into %appdata% on your computer and go to your .minecraft folder. Then go to the saves folder in .minecraft and you need to make your world into a zip file (if you don't know how to do that google it). After that, you need to upload the zip file onto something like mediafire (It's the easiest one I found). After that you make a thread on minecraft forum and paste the link from the mediafire, so people can actually download the map. I also suggest that you include some pictures of the map in the thread.