Many people have heard that the 'c' in Spanish is lisped and is pronounced as a 'th'. So should Barcelona be pronounced 'Bar-the-lona' or 'Bar-se-lona'?
See also: 100 Things to Do in BarcelonaMany English speakers attempt to sound 'more Spanish' by pronouncing the 'c' in Barcelona as a 'th' and saying 'Bar-the-lona'. But in Catalan (the langauge of Catalonia, where Barcelona is found), the 'c' is pronounced as a 's' - so 'Bar-se-lona' is the correct pronunciation.
Incidentally, the abbreviation 'Barça' applies only to the football team. When abbrevationg Barcelona, locals say 'Barna' instead.