The medical System in Spain could be public or private. If you would like to get a job in the public system you should sing up in “la bolsa”, which is a list that hospitals uses to call on the nurses and hire them.
Here is the website of “atria jobs” that will help you to register yourself as a nurse in “la bolsa”.
Contact them at firstname.lastname@example.org if you need information in English.
On the other hand, to get a job at the private sector, you would need to apply directly to the company.