On the first map djp_syd gave the link for, you'll see a grey line that goes from Estació Sants to Plaça de Catalunya.
That's one of the Rodalies lines. It costs the same as a Metro ticket, meaning you can use a T-10 on it. Or, if you're arriving on a long distance train, your ticket will have a "Combinado Cercanias" code on it. (Rodalies is Catalan, Cercanias is Castillian - they mean the same thing). You can put this code into the ticket machine and get a free ticket for this trip. Then ask which platforms have the trains that go to Plaça de Catalunya (many trains will, but they will leave from the same set of platforms - you can get on any train going in the right direction).
Ask for help at the station - I found people very willing to help me (I was headed to the Passeig de Gracia station, which is on a different Rodalies line from the Plaça de Catalunya station).
You can also take the metro as djp_syd describes, but the Rodalies is easier, particularly with luggage.
If you are using a smartphone you can download the Rodalies app. I found it very useful for these trains, although it is only in Catalan.