The RATP (Regie Autonome des Transports Parisiens) coordinates an efficient network of subways, buses, and commuter trains in and around Paris. For info on RATP services, contact their Bureau de Tourisme RATP, Place de La MAdeleine, 8è (tel : 01.40.06.71.45). A english speaking representative is usually available.

If you are only staying in Paris for one day but expect to do a lot of traveling, consider buying a metro pass.
At 65 F for two days, 95 F for three days, and 150 F for five, you probably won't get your money's worth with the Paris Visit tourist tickets, which are valid for unlimited travel on bus, metro and RER, and which give discounts on sightseeing trips, bicycle rentals, and more.
A more practical saver-pass is the Formule 1, for 28 F per day, you get unlimited travel on buses, metro, RER within Paris. If you are staying in Paris for more than a few days, get a weekly (hebdommadaire) or a monthly (mensuel) coupon, which allow unlimited travel (starting the first day of the week or month) on the metro, RER and buses in Paris. Both of these must be accompanied by the ID-style carte orange. To get your Carte Orange, bring an idea photo to the ticket counter, ask for a carte orange and then purchase your weekly or monthly coupon.