The following form allows you to generate a pk3 that you can use as a template for your map. These files are the minimum you'll want to have in a map package you plan to distribute. Learn more about how to create maps on the Xonotic Wiki.
A pk3 is a renamed zip file, you should extract this to a folder name "my_map.pk3dir" in your ~/.xonotic/data folder and open the .map in NetRadiant. When you're ready to test, compile the map (this will create the .bsp) which you can play in game.