From the airport:

By taxi: the regular taxis from the airport take about 15-25 minutes. The approximate cost is EUR 25.

By bus: the "Aerobus" leaves the airport terminals every 12 minutes. It takes 20-30 minutes. The cost is about EUR 3.50 per person. It is fast and comfortable, with place for luggage. You must stop in Plaça Catalunya and then transfer to the metro to Paral·lel.

From Gerona airport:
(105 km North), the best option is the direct bus to Barcelona's city center. Once there take metro up to “Paral·lel” station.

By train: you must stop in Sants Estació or Plaça Catalunya and take the metro up to Paral·lel.