Public transportation is quite good in Paris. There are 5 major modes of transportation:
*Metro (Lines 1 - 14, close by lines to IHP are 7, 10)
*RER (Lines A- D, with line B going to the Charles de Gaulle Airport and coming close to IHP)
*Bus (The bus system is quite dense. Dangers are of course traffic, and the fact that bus stops can be quite difficult to find. On the plus side, they let you see a bit of the city.)
*Tram (Lines 1-3; these skirt the outer-parts of the city, close to the périphérique)
*Velib and Autolib: The capacity to rent a bike (velib) or a car (autolib) by the half-hour (must sign up for service).
The main webpage for RATP is http://www.ratp.fr/ and in English http://www.ratp.fr/en/ratp/c_5000/accueil/ . In the upper right hand corner you will find the capacity to look at other versions in English, German, Italian, Spanish, Dutch, and Japanese. However, on these two main pages, you will find the capacity to get specific instructions on how to get from point A to point B (under Travel in Ile-de-France).
If you are staying for an extended period, it can be worthwile to get a navigo card. This is a card that you can charge for weekly use (Monday through Sunday), or monthly use, but you must have some kind of proof of where you live and ID to get it. For example, a rental agreement, or a utility bill can be used or proof. Also, you can ask a friend with such proof to write an attestation d'hebergement on your behalf (see an example here http://www.aide-courrier.com/Attestation-d-hebergement ). You will then need this, a copy of their ID, plus, your ID to get the navigo card. Here is more information (in French): http://www.ratp.fr/fr/ratp/c_21137/forfaits-navigo-mois-et-semaine/
Finally, if you just buy regular tickets to use on the metro-RER-bus-tram system, you will get a slightly better deal if you buy in multiples of 10.