Hi, I travel quite regularly to La Liga games and has already been indicated unless it is big game or at a very small ground tickets are generally available. Under no circumstances buy from an agency. These seats are vastly over priced and never a great seat. If you know the game you want to attend contact the Real Madrid ticket office, you can do this on-line via their official web site. The site can be viewed in English and you generally get a quick response.
http://www.realmadrid.com/portada_eng.htm
If you want to buy tickets when you get to Madrid the ground is not to far out of the City, just a few stops on the Metro and the ticket windows are open business hours. You can buy tickets on the day of the match however you will find the office busy right up to kick-off time.
If you don�t speak Spanish write out your text in English and put it in the following link and this will translate it in to Spanish for you.
http://www.freetranslation.com/
When I go to the Bernabeu I always sit in the middle tier behind the team bench, great views, but beware of the cashunuts shells!
Good luck
GD