To everyone trying to navigate the Paris public transportation system, check out http://www.parisbytrain.com. This website will explain step by step, photos included, what you need to do to get to where you need to go. It also has many useful links, maps, and a forum.
Parisbytrain.com combined with http://www.metro.rapt.fr/ (mentioned by NYCUWS10025 below) will get you everywhere you need to go in France and help you understand the system.
You can access the sites from any computer or from your iPhone/wireless mobile device while in Paris. There are also great free and inexpensive applications (such as Metro/Tube maps) on iTunes that you can download to your iPhone and use while in Paris. Some of the applications are usable without having the WiFi turned on so that you do not incur roaming charges. Good stuff!
Try this site as well http://metro.nanika.net/<</a>/p>
It is available for download to a Palm Pilot and includes not only Paris but other major cities in various cities around the world.