Get the Cercanías Renfe at Plaça Catalunya station. There are trains to Mataró like every 10 minutes everyday.
If you'd wanted to go to Pont Aeri, however, you'd have to go to Passeig de Gràcia station; that's where the trains destinated to Sant Celoni (Pont Aeri's location) stop.
Both trains are from Cercanías Renfe, it's our short-medium distance train line.
You should check this site out, it lets you see the schedules for everyday Cercanías trains in the area of Barcelona and also gives you advice for when and which train to take if you want to get to your destination in a certain hour.
http://www.renfe.com/EN/viajeros/cercanias/barcelona/index.html