I'd highly recommend free walking tours with Runner Bean. An excellent way to get familiar with the city. I went on 2 of them (reserved my spot in advance) - one for La Sagrada Familia and one for Gaudi. You just pay them what you think is fair. Keep in mind you don't actually go into any of the exhibits while on the free walking tour. I returned to La Sagrada Familia on my own the next day having booked tickets online because I didn't want to wait in the ticket line.
La Ramblas is full of tourists but it's worth it to take a little stroll. Be aware of pickpockets!
Here my travel blog for my solo trip to Spain - Barcelona is one of the cities I visited. I've posted pictures, my itinerary, and recommendations.
http://fanofblank.blogspot.com/2014/01/solo-travel-in-spain.html
Enjoy trip planning!