It depends on what kind of device you have. If you have Android, copy the map from sdcard\games\com.mojang\minecraft_Worlds\*your map* to your desktop (or wherever). If you're on iOS, get iExplorer and navigate to the folder that has your map in it and copy that to the desktop (or wherever). Once you have the folder, compress it to a .zip file (get WinZip if you need it). Then go to a file hosting website (like MediaFire or DropBox) and upload the .zip file. It'll give you a link to the file, copy that link and paste it into your post. You'll also want to include screenshots with your map, upload the pictures to Imgur and post them too.
how do i put up maps to download