It's a few months since I was last in Paris so things might have changed, but from what I remember: You can buy a Navigo card from the buffet car on the Eurostar train (I think they used to sell them in the terminal in St Pancras as well, but not any more) which is preloaded with two single journeys. Within the Paris cental area (roughly within the Peripherique motorway) you can use cards on RER or Metro services. Beware at La Defense which has a Metro station and a separate RER station, the latter is not within the inner Paris area and costs more. Alternatively you can use a machine at Gare du Nord to buy a Navigo card and preload it with N single journeys. The machines can be set to use English and are fairly easy to use, but not all machines will sell Navigo from scratch, some just allow you to top up one you have already. We did in fact buy our Navigos on Eurostar for simplicity, but later found we could have saved a euro or two by using one if the machines, which didn't have a queue. As someone else said: at present you only have to touch in within central Paris, but if you use the card to travel further out, e.g. to an airport, your card needs to have more credit on it and then you have to touch out as well at your destination.