When you start up Scribus, it presents you with a dialog for creating a new document. In case you may have closed this dialog, you can bring it up again with the menu File > New.
The document you are going to create will have the following characteristics:
Other parameters should stay at their default values.
Click on OK to create the document.
Here you see, how your document appears in the main window:
Now, it is the time to save your work. Create a directory Scribus-Gutenberg inside your Documents directory, then save the document as gutenberg.sla (use File > Save)