The days when simply being a native speaker of English was enough to get you a TEFL job are long gone. Nowadays you need to be trained and qualified.
The Cambridge ESOL CELTA Qualification or Trinity's TESOL Cert are the most widely-recognised qualifications, and either one will improve your prospects considerably. You should also remember that your chances of getting a TEFL job in Spain are significantly improved if you have a degree, have some teaching experience and if you speak at least some Spanish. Don't forget that there are separate Young Learners TEFL Qualifications which will vastly improve your prospects when applying to schools where the majority of students are young learners.