The first thing we did when we arrived in Barcelona was buy a two-day ticket for the Bus Turistic. A hop on, hop off bus that covers most of the city, we figured it would give us a general idea of what was out there. It was a great idea and one we suggested to all our friends when they visited. I highly recommend it.
But beyond the Bus Turistic, my main advice is to get a bus map at any of the subway stations and consult that before you go anywhere. Alan and I simply loved the bus and tram service in Barcelona. It was efficient, clean, and the maps made it very easy to know where to get on or off. I believe all the buses now announce stops. We usually only took the subway to places we couldn’t get by bus.
By the way, bus number 24 will leave you right in front of the entrance to Park Güell. All other means of transportation, including the Bus Turistic, leave you at the bottom of the hill.