The Margins tool is found on the Page tab.
Your visual will default to normal margins. That is a half an inch (1/2") around the page. If you would like to change this, simply select from the Margins drop down menu on the Page tab.
You can select a narrow or wide margin, or define your own custom margin.
Narrow margins reduce your margin to a quarter of an inch (1/4").
Wide margins increase your margin to a full inch (1").
To set your custom margin, you can define the margin value for every part of your page: left, right, top, and bottom.