If I understand what you want to do, from the Format menu, select Paragraph
and change the alignment to Justified.  In Word 2007, that is done by
selecting the Home tab of the Ribbon and click on the arrow at the right and
end of the Paragraph bar.

You can also do this by selecting the text and then hold down the Ctrl key
and press j.

It would really be best though if you created a style in which you define
the paragraph formatting as justified and then apply this style when you
want the text that way.