What do I need to know about gameday in Barcelona?

Hello all! I will be watching El Classico live in Camp Nou. It’s the first time I’m in Barcelona for a game, same for my travelling companion (my parents). I’m wondering what to keep in mind for Sunday evening. For example what time should we be around the arena? Should we be there hours early to enjoy the mood? Or will everyone be there closer to gametime? How is the food close to the arena? Etc. Is there something we really should know about going to a game, and not just any game an El Classico?

Apologies if this isn’t the right forum for this.