Barcelona-Sant to Placa de Catalunya?

Tickets machines in the metro easy to use. You want red line1 in direction of Fondo. We were there last week and it is very warm underground. Taxis are cheap, would cost under eu7. Google Barcelona metro to see a map of the system. FYI .. we enjoyed Tapas 24 in that neighbourhood.

Thank you ... But is the Metro the same as the Renfe train? I am a bit confused by that.

No but it's in the same building. Renfe is Spain's train, metro is part of
Barcelona's transit system.

I would just take a taxi. No worries about getting lost that way. I rented an apartment by the Cathedral in July and was a quick 5 minute walk to Placa de Catalunya.

Arriving at Barcelona Sants, follow signs for Metro Line 3 (green) - direction Trinitat Nova.
After 7 stops you will arrive at Catalunya.
(Red Line 1, as suggested by Marika, does not run through Sants Estacio)

Barcelona can be confusing as you have ...
Metro lines 1-5 and 9+10, identified with "M" inside a red diamond.
Ferrocarrils de la Generalitat de Catalunya lines 6-8, identified with an interlocking chain type symbol.
Rodalies de Catalunya - interurban trains prefixed with "R", operated by renfe.

http://www.tmb.cat/image/image_gallery?uuid=3f54072a-5f82-4eee-a413-bc121dcd8e12&groupId=10168&t=1436527848715
http://www.tmb.cat/en/mapa-metro-barcelona

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.

Congratulations Harold on an excellent job explaining what I didn't even dare attempt