No visa is required to enter Greece for studies if you possess a valid passport from an EU Member State, Iceland, Liechtenstein, Norway or Switzerland.
Applicants from countries outside the EU or the Schengen zone need a student visa. The length of time needed to acquire a student visa differs from country to country. For this reason, prospective students who are requested to apply for a student visa are encouraged to communicate well in advance with the Greek Embassy in their country for information on the procedure and on the time needed for the conclusion of the process. Therefore, we strongly encourage you to submit your application as soon as possible.
For more information, see here and here.
For a list of the necessary documents, see here.
For a list of the Greek missions in your country of residence, see here.