Your question was broad enough to include two subjects,so i'll give you two answers.
1: If by map you mean just a basic map to build/survive on then you should do this...
-->start game-->create new-->start!
2:If you mean a custom map,as I think you do,then this should work.
-->make map on mcpe
-->go into a file explorer
-->navigate to games/com.mojang/minecraftWorlds (sorry,I only know what the directory is for android...)
-->make a zip of the folder for your world
-->upload the .zip onto a file sharing website such as Dropbox
-->copy-pasta the url to wherever you want your map posted