You need to create a template that has the settings you want and then make that template the default. The way you do this is as follows:

Part 1: Create a template

Start with a blank document.
Change whatever settings you need to change. To change margins, modify the Default page style.
Save the document as a template: File > Templates > Save, and specify a name for the template.

Part 2: Make your new template the default

File > Templates > Organize
In the left-hand list, find your template. Most likely, you'll need to double-click on the folder called Default, and your template will be inside that folder. Click on the template.
Click Commands. From the menu that appears, choose Set as Default Template.