European Union Students
The Certificado de Registro is for students of the European Union who will study in Spain. It must be requested within the first month of your arrival, and it is not necessary to renew this card annually.
International Students Non-European
The first thing that you should apply for before you arrive is your VISA. The type of visa that you should apply for depends on the length of stay (Visado de Alumno process and documents).
The Tarjeta de Identificación de Extranjero (NIE) is even more important than the Visa, however you cannot obtain this until you have the 90 day visa. You must apply for this within the first month of arrival and it is imperative that you renew this card annually.
For more information go to Immigration procedures UNAV