There are approximately 15,000 taxis in Paris. Try spotting one. Unlike in other cities they are no one colour in particular. To distinguish a taxi look at the roof, regardless of the car’s colour. If it has a Taxi Parisian sign on it you’re in luck. If it’s illuminated you’re even luckier – it’s free.
There are around 450 taxi stands dotted around the city in Paris. Queuing in line at one of the stands is your best bet for getting a taxi. You can try hailing one from the street, waving your arm as one passes, but they may not stop. Particularly if you are within 50 metres or more of a stand. They’ll drive right past.
Note also that taxis are expensive in Paris. You can keep an eye on the fare by looking at the meter, but extra charges may be added on, if luggage is handled for example.