how do I permanently change the margins on the page that opens by default when I choose "Create: Writer Document?
You need to define a new default template. On an empty document, look for the text “Default style” in the status bar at the bottom of the window and click on it: the style editor will open to modify the default page style. In the Page tab of that dialog you can change page size and margins. After accepting the changes, go to File → Templates → Save as Template → provide a name for the new template → check the “Set as default template” box → Accept. From now all new documents will have the new settings.
You can learn more about the use of styles and templates in the official documentation or, if you prefer a different approach, in my free book about Writer.