The best options for traveling within the urban area of Barcelona are: train, bus, metro and tram.
Visit the following websites:
Next to Plaça Catalunya there is Urquinaona station or the metro yellow line (L4) that leads directly to the CCIB by Maresme-Forum station.
You can also get to the CCIB by bus: lines 7, H16 and tram: Forum station.
All these means of transport leave you just few minutes away from the CCIB by foot.