The FC Barcelona stadium - Camp Nou - can be reached very easily from the city centre by metro. On match days it is not a good idea to try to get to the stadium by.
From the central Plaça Catalunya you need about 15 minutes by metro on line L3. Travel towards "Zona Universitaria" without changing trains till you get to the station "Les Corts". From here follow the signs to the stadium, which is about 15 minutes walk. The Les Corts station is located at the major road Travessere de les Corts. Walk about 500 meters to the west, and then you will see the stadium. The shortest route is when you go clockwise around the grounds until you come to your entrance. On non-match days go to the only entrance that is open, which has the box office for stadium tours.
You can also get to the stadium with the line L5. In this case get off at the "Collblanc" station. Then follow the road north (mountains) until you reach the Travessera de les Corts mentioned above. Then you turn right up to the Camp Nou.
Please note that on match games the metros can be a little full, especially after the game. Plan sufficient time for entrance controls and finding the way to your place (it is the largest stadium in Europe ...). You should be in the stadium a good 30 minutes before kickoff.
Here you will find the Camp Nou stadium on the map.

Camp Nou - Europeans largest football stadium.