Paris taxi drivers are not known for their flawless knowledge of the Paris street map : if you have a preferred route, say so.
Taxis can also be hard to find, especially at rush hour or early in the morning.
Your best bet is to find a taxi rank (station de taxis, marked with a blue sign) on major roads, crossroads and at train stations.
A green light on a taxi’s roof indicates the car is free & a red light means the cab is busy.
There is a minimum charge of €7, plus €1 for each piece of luggage over 5kg or bulky objects.
Most drivers will not take more than three people, although they should take a couple and two children.
There is an additional charge of €3 for a fourth adult passenger.
Taxis are not allowed to refuse rides if they deem them too short and can only refuse to take you in a certain direction during their last half-hour of service (both rules are often ignored).
If you want a receipt, ask for "un reçu" or "la note". 
If you call a taxi you also pay for the time it takes your taxi to reach you. If you wish to pay by credit card, mention this when you order.
Be carefull in the train station and especially at Gare du Nord
There is only one official taxi stand outside the station and this is where you have to go to get a taxi.
Inside the station you will be harassed by many men asking if you need taxi or transportation.
Some of them look quite official, they wear uniform-ish clothes and say they are the station dispatcher.
For your comfort and your security don't follow them !