In comparison to the Premiership, tickets for most La Liga matches are cheaper and more easily available. The main exceptions to this are when the local side is playing either Real Madrid or FC Barcelona, local derbies, and of course, the Clásico when Real Madrid and Barcelona play each other, in which case tickets are not only expensive but also very difficult to come by.
Tickets generally go on sale about a week before the game, with preference for members of the club. Any remaining tickets become available a few days later and can usually only be in person obtained from the ticket office (taquilla). Opening hours tend to be 10am to 1pm and/or 3pm or 5pm to 9pm, but you should check the official websites for more details. (Click on the 'Primera' in the Navigation Bar for all the official websites - Segunda coming soon.)
The traditional time for games to start was always Sundays at 5pm but TV sponsorship now means that matches can kick off at anytime between 6 and 10pm on Saturday and between 5 and 9pm on Sunday. Unfortunately, the final decision as to when the match will begin can sometimes be taken very late and if you're travelling to Spain just to see the game, it would be wise to make a weekend of it.
To be absolutely certain, the best source of up-to-date information is the Liga de Fútbol Profesional website (www.lfp.es). The site is in Spanish but click Liga BBVA in the top left of the screen for kick-off times for Primera and Liga Adelante, just below it, for Segunda. All other competions can be accessed by cliking on the 'Otras Competiciones' tab.
If you like to sit near the half-way line like me, you have two options. The cheaper Lateral is uncovered and may face the sun, whereas the more expensive Tribuna or Preferencia is the main covered stand, where you're unlikely to get the sun in your eyes. Meanwhile, the ends are either the Fondo Norte, the North End, or the Fondo Sur, the South End. If you're not in Tribuna, how close you are to the pitch is designated by the words Alta, meaning high and therefore further from the pitch, or Baja, meaning low and closest to the field of play. In some stadiums you might see Segunda Gradería for Alta and Primera Gradería for Baja.
Ticket touts are a necessarily evil both in sport and at musical and theatrical events, but sometimes, if you've taken a late decision to go to a big game, no other option is available. Beware of being sold forged tickets and always check at the taquilla first - it is not unknown for touts to tell punters that tickets are sold out when there are plenty available at a considerably cheaper price.
Ticket Agencies add a considerable mark-up onto the price of a normal ticket. However, if you can't physically get to the ticket office and want to be guaranteed a ticket, particularly for the big game they are definitely the safe option. Most also will organise accommodation as well. I will be checking various ticket agencies comparing prices and service and will post a recommendation here soon.