Amsterdam is located 15 minutes by train from Schiphol Airport. Trains go every 15 minutes to Amsterdam Central Station, where you end up right in the middle of the city centre.
There even go frequent trains at night. Train tickets can be retrieved from machines at the airport. From Central Station, you can take the tram, metro or taxi to your hotel. You can even walk, when your hotel is very nearby.
There is a fixed fee for going from Schiphol Airport to the city centre by taxi. However, it's quite expensive (35 euro).
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