Not an easy question to answer . . . "when" is an important variable . . . . there have been periods of kings, republics, city-states with a variety of governmental forms, and of course the origin of democracy (centuries ago and again today). Pick a period and your text should help you narrow your answer . . . . .
Greece has a parliamentary republic political system. It is based on the separation and balancing of executive, legislative and judicial powers. The highest law in the Republic of Greece is the Constitution.
Ancient Greece wasn't a unified nation, it was a collection of city-states who went to war with each other all the time. They had different forms of govt, usually monarchies. But I think you're looking for Athens, the first democracy.