To get walking directions, enter your to and from destinations as usual. On the upper left side of Google Maps, just below your to and from addresses, there's a drop down box to select your mode of transportation. This is where you'll indicate that you're walking.
Not all locations have all options, but the three choices for transportation in Google Maps are by car (the default), by public transit, or by walking. If you select public transit, Google gives you walking directions to and from the bus or train stops required to get to your destination.
Walking directions are also available on some brands of cell phone with Google Maps for Mobile.