How do I create a line border in Microsoft Word 2010?

Hi

To only add a line between paragraphs or the contents, you can insert it from the shapes and after that change its formatting like (weight, color, shadows, ...etc) from the format tab.

To add a border around text

 Home menu option > Paragraph group > Boarders > Outside boarder

for Table, click anywhere in table

Design menu > Boarders group > Boarder > Outside Boarders

Without knowing what exactly mean by line border, In MS Office Applications, you can create standard and custom borders from Home>Paragraph tab in Ribbon.