"Planning" in Barcelona

For my upcoming trip to Barcelona, I am booking tickets for the main sights in advance. Is there a single website from where I can buy advance tickets for all important attractions Barcelona? ... advance, for many Barcelona attractions. What is the best time slot for each main Barcelona attraction?

Best time of the year to visit Barcelona? We will be visiting Barcelona as a family. We can do that in January or defer the trip to any other month, ideally when the crowds are less, attractions are open and it is cheapest to fly to Barcelona. Any suggestions?

Traveling as a couple to Spain with 12 days to spare. We wish to include Barcelona, Madrid and Andalucia in our Spain adventure. We are starting from Barcelona. Need an indication on how many days/night to spend in Barcelona.

Which apps to download before embarking for Barcelona?

Low cost airlines seem to fly to other airports - Girona and Reus. How easy is it travel from these airports to Barcelona?