From Athens airport you take bus X96 to Piraeus, the port of Athens, and from there the boat to Poros or you rent a car and drive to Galata and take the ferry over to Poros. See the map below. Read more about the trip to Poros under Travel information.
Poros town is located on the small island. Most of the hotels and all beaches are located on the Big Island within walking distance of the town.