Using software is the easiest way to make Google&Yahoo site maps. Their are many free software you can find around the Internet. Search with the word "Free Site Map Builder" and then you will find a lot of them.
However it's very well if you can create site maps by yourself because you can optimize them by the way you want. First time you may use free software create one for your sample, see what it look like and follow them.
Hopefully this is useful.
R. Apichart
did you mean a Google site map ? if those the site which mentioned by member above will make it for you automatically. register at Google webmaster tools, it will help you a lot.
Just signed up to Google webmaster tools - its great... Thanks for the tip!