Every major city has them – the “Hop on Hop off” Buses. Barcelona has actually two companies offering good services covering the whole city. I have no personal preferrence or recommendations to give – I believe both companies deliver a good product. You might decide based upon the itinerary they offer. When I have visitors I usually suggest them to take the Hop on Hop off Bus on the first day in town, since this way you get an overview of what is there to see and to do. Sometimes, especially during high season and weekends, you can observe long lines to get on the buses. Maybe try to avoid weekends if you can.
- Barcelona City Tour: http://www.barcelonacitytour.com you can buy tickets online or find the Bus station at Plaza Catalunya (opposite Café Zurich).
- Barcelona Bus Turistic: https://www.barcelonabusturistic.cat The same here, either buy tickets online or get them at the booth at one of the main stops like in front of the El Corte Inglés at Plaza Catalunya.