I hope this can help:
Civil registration began in Spain in 1871 (before that most records were kept by the church priest at the local parish, though today many of the baptismal, marriage and burial documents are kept in more centralised diocesan -i.e. seat of the bishopric- archives).
Access to BMD records in the Civil Registry in Spain is free of charge, but it does imply some difficulties for the newbie researcher. BMD records are kept in the "Registro Civil", and each town council/municipality has its own Registro Civil. Their services are slowly becoming digitised, which means documents can be ordered online via the website of the Ministry of Justice (Ministerio de Justicia:
http://www.mjusticia.gob.es/cs/Satellite/es/1200666550194/DetalleInicio.html). Scroll to the end of this post to see specific steps to take.
If you are looking for someone's birth certificate, for instance, you need to know in which town or city the person was born (as registrations usually took place in the birthplace). Note that in Spain some provinces take the name of the capital city of the area, so Madrid can mean the city, the province or the region of Madrid! Moreover, certain large cities in Spain (namely Madrid, Barcelona...) are divided into districts, so when ordering the certificate you should specify the district, the street or the hospital where the birth took place. If the birth took place in, for instance, the city of Barcelona, you will need to specify this information.
The date is often less of a problem, as registrars and civil servants often look a year or two either way if the place is positively correct. They will not look over a wide range of years unless they are in a very small town (or happen to have a lot of time on their hands).
I think, for starters, this should be enough. If you are searching for pre-1871 information let me know and I can tell you about searching in the church records. If you have any problems with the language let me know!
To order a birth record, for example, you will have to go here:
http://www.mjusticia.gob.es/cs/Satellite/es/1200666550200/Tramite_C/1214483947232/Detalle.html; click "Sin certificado digital" at the bottom and then the "Sede electrónica" button. Of the three options you get, click "Solicitud de certificado de nacimiento". You will have to select the region, then the municipality/city and fill in the gaps accordingly. I'm afraid the language can often be a barrier, but being a free service we shouldn't complain