Take the train from T2 to Sants Estació (there is a free shuttle from T1 to T2). There is a train every half an hour which takes twenty minutes to arrive to the city center.
If coming by taxi, ask for the UPC CAMPUS NORD and expect a fair near 25 euros (including the extra charge for airport service and one suitcase).
The new metro brunch, line 9, connect T1 and T2 with Zona Universitaria in ~30min.
By bus (Aerobus),the line from T1 and T2 to Catalunya Square takes ~40min (every 10 min). Tickets at the bus.