Make sure you get your European Card, it’s free of charge and you can have it issued from your local Social Security Office (in Donosti, at Pio XII and Riberas) ,also online:
In Basque: https://sede.seg-social.gob.es/wps/portal/sede/sede/Ciudadanos/asistencia%2520sanitaria/06asistencia%2520sanitaria?changeLanguage=eu
In Spanish: https://sede.seg-social.gob.es/wps/portal/sede/sede/Ciudadanos/asistencia%2520sanitaria/06asistencia%2520sanitaria?changeLanguage=es