Exact match times are announced 10 days before the round of fixtures takes place so if you are booking far in advance for a Liga game then make sure you are in Barcelona in time for a potential 20:00 Saturday game as there is no way of knowing which games will be switched.
As a guide, if Barça are playing in the CL in the following week then the previous league game will be on the Saturday and if the CL game is on Wednesday the following league game will be on Sunday (this is right 90% of the time but is not an infallable rule). For games when there is no CL game the following week it is impossible to know if a game will be on Saturday or Sunday until the fixtures are announced 10 days previously.
For all but the biggest games you will be able to buy tickets on the day from the ticket booths at Acces 19 of the Camp Nou which is a 5 minute walk from Les Corts metro station (Green line "L3"). For nearly all games it is next to impossible to get more than two seats together.
If you want to buy tickets in advance then non-members can do so from a month before the game in question via the FC Barcelona website http://www.fcbarcelona.com/web/english/
To collect tickets which you have bought over the internet, you only need your credit card which you used for the transaction and a form of identification, passport, national ID card or Carnet de Soci will do fine. The ticket booths for collections are at Accès 9 of the stadium, behind the shop and next to the palau blaugrana.
Alternatively the official website now gives you the option of printing your tickets at the time of purchase, because access to the Camp Nou is controlled by barcoded tickets it's possible to print off a pdf file with the barcode and details of where your seat is located. Tickets are not posted out by FCB.
The Camp Nou
For the best atmosphere you should try and get a seat in the first or second tier of the lateral where a lot of the singing takes place. If you sit in gol 1/2 then the atmosphere is good but not quite as lively although you will be nearer the Ultras (Boixos at Gol Nord in limited numbers nowadays and Almogavers at Gol Sud who make most noise throughout the game).
Prematch drinks are best taken at Bar La Jarra just opposite the south end of the mini estadi. €3 buys you a pint of Estrella Damm, the local beer, and they also do a good range of tapas to soak up the prematch alcohol. The bar contains a good mix of culés and some Boixos Nois but there is rarely any trouble to my knowledge.
To get to the Camp Nou you are best off using the city's metro system which is superb. If you are sitting in Gol Nord or Tribuna then the best option is the green line (line3) to Maria Cristina. Elsewhere you are best served by Collblanc on the blue line (line5) or Les corts which is also on the green line.
Barcelona, the city
Stay in a hotel if you wish but I will leave other people to recommend places. Barna also has a decent range of hostels ranging from the clean and smart to the Kabul.
For a nice spot of lunch i can reccommend the bars at the bottom of the Mercat de la Boqueria which do a great range of fresh grilled seafood. It is a lot easier if you speak Spanish or Catalan here though the staff do speak limited English and are evry helpful.
There is also a great range of tapas bars in the Born area of the city but if you want something overpriced then the Rambla will serve your needs well. There is a Hard Rock cafe at Placa Catalunya if you want something microwaved and American or Burger King/McDonalds/KFC on La Rambla if you fancy digesting something that isn't really food at all.
In terms of tourist attractions then don't miss the Olympic complex at Montjuic, the Sagrada Familia, Casa Batlló in Gràcia (famous Gaudi building) and Parc Güell. There is a lot more besides.
Great travel guide to Barcelona by Wikipedia. got lots to read like places to visit, good bars, restaurants, hotels and general tourist sites.