To take courses at any university in Spain, you should contact the secretary's office of the university that you are interested in. If it is a public university, you should follow the usual pre-enrolment procedure for the relevant autonomous community. You can consult the list of Spanish universities.
If you want to embark on a course of studies at a foreign university, you should contact the relevant Consulate or the different cultural organisations that many countries have in Barcelona, such as the Institut Français de Barcelone, the British Council, the Goethe-Institut, the Istituto Italiano di Cultura di Barcellona or the Institute of North American Studies, where they will provide you with information on the qualifications required for university admission and any other additional requirements.
If you are enrolled at a Catalan university and wish to continue your studies at a university outside Catalonia, contact the international relations office at your university, where you will be able to find out about this subject.