Getting from the Orly Paris airport to Gare St Lazare train station

Judith - You din't say how much luggage you'll have or what time you'll be arriving, but here goes. Take the RER C from d'Orly Airport(direction Pontoise) to Bibliotheque-Francois Mitterrand station. From Bibliotheque-Francois Mitterrand take Metro Line 14 (direction Saint-Lazare) to Saint-Lazare. You can also check out parisbytrain.com. EDIT Thanks Tim for the assistance.

There is one step missing from Joel's directions. First, take the airport shuttle bus to the Pont de Rungis Aeroport d'Orly RER C station. Then take the RER C to Bibliotheque-Francois Mitterand.

Judith, I suggest you consult a Metro/RER map before departing Orly. There is a shuttle of some sort to the Antony exchange from Orly airport. My map shows RER B as the connection from Antony to central Paris. From the Chatelet exchange in central Paris, I would transfer to Metro Line 14 toward Saint-Lazare for Gare Saint-Lazare. According to RS, there is a TI at Orly. You can ask for directions there after your flight.

The shuttle to which Swan refers is the Orlyval. It's a driverless train that goes between the airport and the Antony RER B station. Taking the Orlyval plus the RER B and metro is a bit faster than taking the airport shuttle bus plus the RER C and metro which Joel and I suggested, but it's more expensive (€8.30 for the Orlyval plus €2.45 for the RER and metro as compared to €2.50 for the shuttle bus plus €3.95 for the RER and metro).

The OP was regarding "which trains to take."