The agora is a large, park-like area just to the northwest of the Acropolis.
There are two primary entrances to the agora: South (for those who are coming from the Acropolis) and North (closest to the Metro stations). You’ll find the South Entrance as you leave the Acropolis (heading north), pass the Areopagus (on your left), and walk downhill along the route of the Sacred Way. Easy.
For the North Entrance (recommended), you need to take the Metro to either Thissio Station (Line 1) or Monastiraki Station (Line 1 or 3). The North Entrance is located between the two stations on Adrianou Street. Adrianou dumps right into the Thissio Station … just step out of the station, find Adrianou, and walk due east for about 50 yards. The entrance is on your right. From Monastiraki Station, walk a block north (on Areos Street) and then turn right on Adrianou. Go about 100 yards and the entrance will be on your left.