Actually, Spain has long been at the forefront of legislation regarding hygienic standards for tattoo parlors, so compared with many other countries, odds are good that the place you pick will be clean. (Obviously, any law is only as good as the people who obey it, and just as largely unregulated countries like the US have many good, clean shops, there are a number of fairly disgusting places here.)
Frankly, you choice of a shop should be determined to a great degree by the style of tattoo you're after. Feel free to send me a PM and I'd be happy to orient you a bit.