Alex,
You need to visit your university library in Spain and make an appointment for a literature search in your area of concern with one of the professional librarians.

You will need to obtain books and journals in your area, aschew newspaper articles since they are not permissable usually in dissertations.

Also, do you have access to electronic databases at your university? If you use these, generally these are peer-reviewed and are considered acceptable in a dissertation.

You need to decide the conceptual as well as the theoretical framework for your dissertation then narrow in on your topic.

Good Luck,
Booklady