Getting to Central Paris from the Airport

From Roissy Charles de Gaulle Airport

This is the main Paris airport; flights from America will arrive here.

By taxi: ~50 euros.

By rail: ~8.40 euros. From terminal 2, follow the signs to the RER (regional rail) station. From terminal 1, take the free CDGVAL tram to the 'Roissypole' or 'Terminal 2 Gare' station. You will need to buy a ticket from the ticket counter unless you have a European credit card or lots of euro coins. The train takes ~45 minutes to central Paris where you can connect to the metro system. In Chatelet-Les Halles you can transfer to Metro line 14. In Saint-Michel/Notre Dame you can transfer to RER C. Keep your ticket since you will need it to get out of the system when you arrive.
[Detailed Instructions]

By bus: Air France runs a bus "les cars" from the airport to central Paris for ~16 euros. Take line 4 to Gare de Lyon or Gare Montparnasse, from which you can walk, catch a taxi, or take the metro to your final destination.

More information

From Orly Airport

Many discount airlines within Europe use this airport.

You can catch either the OrlyBus to Denfert-Rochereau (~6.40 euros) to connect to the metro system, or take the OrlyVal shuttle to the RER B train to central Paris (slightly faster, costs slightly more). Air France and other private companies also run shuttle buses which may be of interest if they are going to near your hotel.