The Athens Familiarization Walk (from GreeceAudioGuides.com) begins at Syntagma Square. Bus #95 from the airport terminates there. Chances are, this is the first place you saw when you arrived in Athens. Two Metro lines (2 and 3) stop at the Syntagma Station.
If you arrive by Metro, follow signs to “Syntagma Square” up to the surface. Find the fountain in the center of the Square. Press “Play” on your MP3 player. It’s that easy.