Assuming that you hold a British passport, you don't require a visa to work in Spain. (EU citizens are free to live and work in any of the 27 member states. There are some work restrictions for citizens a few of the newer East European member countries but they don't apply to anyone from the UK).
You'll obviously need to speak fluent Spanish for most types of employment in Spain, otherwise you'll be limited to things like bar work in those resorts which mainly attract British tourists, or the types of jobs that immigrants (with poor English skills) can find in this country. (e.g. factory production lines or agricultural work).
Simply entering 'jobs in Spain' into Google will find loads of agencies which claim to be able to find you work. Otherwise, the best way to find work would probably be to take a holiday to Spain in order to find out how the locals go about finding employment.
There's some advice on relevant issues, from the British Government, here:
http://www.direct.gov.uk/en/BritonsLivingAbroa d/index.htm
Chris