Save this number in your phone if you take taxis:
Taxicentrale Amsterdam: +31 020 777 77 77.
It is the biggest cab company in Amsterdam. They accept credit cards, function 24 hours a day, will pick up people all around Amsterdam/Schiphol, and their cabs are honest. (Police/Emergencies: 112)
There is an official taxi line outside. Taxis from Schiphol to Amsterdam city centre should cost 45-55 euros with an optimum route. If it’s above 70 euros, ask your hotel or hostel for assistance. I’ve been ripped off myself… I only use Taxicentrale now.