Margins in Word create a buffer space between the edge of the page and the document content. Some style guides (e.g. APA, MLA) require a different amount space in the margins than others. Changing the margins every time can get annoying. To save time, change the default setting for margins.
To set the default margin space, first open Word and navigate to the Page Layout tab. Click the Margins button, which will activate a drop down box; select Custom Margins at the bottom. The Page Setup dialogue box will appear. Here you can set or select your desired margins or page setup, and select Set As Default. Click Yes to apply the settings to the current document and all subsequent documents you create. You word documents will now set to your desired margins every time.
For a step-by-step guide to change the default margin size in Word, visit this Knowledgebase article: http://helpdesk.etown.edu/helpconsole2012/KB/default.aspx?pageid=change_default1_margins.