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 email@example.com 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.