If you prefer to take the Express X96 bus to Piraeus, you need to leave through exit door 5 (turn right out of customs, the last door at the end of the arrival hall).
You need to purchase a ticket from the booth before boarding the bus.
Tickets cost 5 Euros per person (2013). The X96 bus and the bus stop are clearly marked in English and Greek with the destination (Piraeus).
There's plenty of space for luggage and the buses go at least twice an hour, 23 hours a day. When you board please make sure you validate your ticket in the red box on the post behind the drivers seat. There are on the spot fines if an inspector finds un-validated tickets! The journey time on an average day takes approximately 1.5 hours at peak times.
On your return journey, the bus stop is no where near where you will have been dropped off on your incoming journey, which is a little confusing.
Disembark from the dolphin or cat and then walk left along the harbour road passing the waiting room, public toilets and Port Police offices (which will all be on your right) and then exit Gate E8. Walk along the pavement to the lights and then cross the service road so the you are heading away from the sea with the harbour at your back. The pick up stop for the X96 will be the first one you come to, and the ticket office is right next to it.