Best option is by “Aerobus” costs around 5€ and takes you directly from the airport to the city centre (in fact has 3 stops in the centre: Pl. Espanya, Urgell, Pl. Catalunya)
You also have metro but, believe me, it’s only worth if you go to the same line (purple).
Taxis are affordable too. It costs 30€ and if you are more than 2px it’s a good option.