The easiest way to do that is using Printer Properties--begin with File -
Print (or Office - Print, if you're using Word 2007), and click Properties.
The location of the settings vary by printer driver. For my current printer,
in Printer Properties, there are tabs across the top. In the one labeled
"Main", there's a Grayscale Printing checkbox. You very likely have
something equivalent, and possibly in the same location.