When traveling within Paris city limits (Paris Intra Muros) and the close suburbs by all means use taxis, metro, RER.

When traveling to the greater Ile de France, first check RER, SNCF trains, and in last resort rent a car. For example, Versailles is easily accessible by public transportation, Giverny and Monet's house not that easy.

By lifting your arm as they approach you can hail a cab along any street as long as you are more than 50 meters away from the taxi stand, if not go there and take the first one. The cab is available for hire if the green lights on the roof are lit. Red of course means occupied.

There are different tariff zones in Paris and the meter changes the rate as the taxi enters the new zone, the farther the zone the higher the rate. There is a night and a day rate. All this information: free/occupied, tariff zone, night/day is diplayed on the taxi sign on the roof of the car as this is a very reglemented profession and the police can check that the appropriate rate is being applied.

Your first piece of luggage is free