Your greatest asset when it comes to finding a job in Spain will be an ability to speak Spanish. Even if you're not fluent in the language, mastering the basics and being able to hold at least some kind of conversation in Spanish will greatly increase your chances of finding work. Those who speak English, Spanish and German are usually able to jump straight to the front of the job queue in popular tourist areas.
Wherever there are large numbers of tourists and ex-pat residents you'll find there's a big demand for people with language skills to work in shops, bars, restaurants, estate agencies etc. If you're a master of two or more of the main European languages you can be fairly certain of finding work in the tourist industry, teaching or translation services.
It's not impossible for non-Spanish speakers to find work but if you're linguistically challenged you'll need to seek job opportunities in those areas dominated by communities of your fellow countrymen. It's easy enough for young people to find seasonal work (bar work, cleaning holiday villas, holiday repping etc) but it's usually hard graft for poor pay and of course there's no job security. Some youngsters pitch up in Spain at the start of the season, take on a bar job and master the language simply by daily contact with Spanish colleagues and customers. When the seasonal work runs out their newly acquired language skills give them a wider choice of work opportunities.
There are thousands of summer jobs for Brits in popular resort areas such as Benidorm on the Costa Blanca, Lloret de Mar on the Costa Brava, Torremolinos on the Costa del Sol and Magaluf in Mallorca. But don't expect to learn any Spanish in any of those places which are more British than Blackpool!
If you're looking for something more permanent it's a good idea to get a TEFL (Teaching English as a Foreign Language) course under your belt as English teachers are in big demand in Spain.
EU nationals don't need a work permit in Spain and have the same employment rights as Spaniards. Many foreigners have been able to establish successful businesses, in most cases targeting their own countrymen in the popular tourist and ex-pat areas. Mechanics, bar owners, restaurateurs, B&B owners, nurses etc have all found they can make a comfortable living without ever having a Spanish customer!
If you plan to be self-employed or start your own business, brace yourself for some frustrating delays while you wade through all the necessary paperwork and bureaucracy. It's best to hire the services of a Spanish "gestor" (a kind of Mr Fixit who specialises in Spanish bureaucratic systems). He or she will be able to sort out the paperwork for you and make sure you're complying with all the regulations. You'll find a "gestoria" in every major town in Spain and in areas where there are big ex-pat communities you can normally find at least one gestor who speaks reasonable English.