Creating a border:

Select the cells where you wish to apply a border.
From the pull-down menus, select Format > Cells... then click on the Borders tab.
In the Line Arrangement section, under Default, click on the icon that best shows the style of border you wish to use, or
Under User Defined, select custom border styles by clicking on the individual lines shown bordering four 'dummy' cells.

Changing the width of a border:

In the Line section, under Style, click on the desired line for your border, and
Under Color, choose the desired color for your border.