There are seven different sites where you can find taxis waiting to take you into the center of Madrid. In Terminals 1, 2 and 4 there are separate taxi stand located by the arrivals and departures areas, whereas in Terminal 3 there is a single tax stand located by the arrivals area.
Once you have landed and cleared passport control and retrieved your luggage, look for the clear signs inside the airport directing you to the taxi pickup areas. If you are unable to find these signs, simply ask an airport staff member to direct you to the taxi pickup areas.