From Arlanda, you can either take a taxi, use the Arlanda Express fast train service or go by the airport coaches to get to the city center.
If you use a taxi, the advice is to only use the three major taxi operators (Taxi Stockholm 15000, Taxi Kurir, or Taxi 020 – they have the lowest rates and fixed prices to and from Arlanda – in order to avoid surprises). There are of course taxi stations at both Arlanda and the Central Station.