The best way of reaching Pla a Catalunya, which is the center of the city, is by bus. You can buy the ticket on the bus.
Buses leave in front of every terminal.
The Aerobus leaves the airport every 15 min. Pla a Catalunya is the last stop. By taxi there are taxi stops in front of all the terminals. The trip to the centre won't take you more than 20 min, depending on the traffic. Expect to pay about 2.500-3.000 pesetas.