If you like football, you should experience a FC Barcelona match.
It's great fun. The atmosphere is amazing.
Get there at least an hour in advance for a better feeling of stadium, the ambiance, a visit to the store, museum, etc.
Purchase your tickets directly on the FCBarcelona website. Unless you're in Barcelona for an exceptionally important game, you can buy the tickets just two or three days before the match kick-off. The longer you wait, the more the seats that become available, as these are released by the 'socios'; especially when trying to find several seats next to each other, you have better chances of finding what you're looking for by checking the site regularly.
Bring your own sandwich and have your snack-dinner at half-time just like most Spaniards do...
A Barca live game: a wonderful experience.