Greece belongs to the so-called Schengen Area of countries that have agreed to allow free movement of their citizens within this area as a single country.
Citizens of certain non-Schengen countries are required to obtain a Schengen Visa (also known as a Short-Stay Visa) in order to enter the Schengen Zone. To determine whether you need to obtain such a visa, please check the website of your own embassy for more information.
If you need a visa, you must apply for it from the Greek embassy or a Greek consulate in your own country.
Relevant information on how to proceed can be found at these websites: