You can reach city center by metro, suburban railway by bus , taxi and car hire. If you decide to use the suburban railway and you are coming from the airport, you will have to change at Nerantziotissa station and take the metro, there is a connection there. Metro operates 05.30 – 24.15.
A nice way that can get you to Athens center from the airport is the city bus. There are 4 bus lines X93 to Kifissos ( the station where toy can take busses for the rest of Greece). X95 is the Bus that will get you to Athens center ( Syntagma square).
If you would like to take the ferry, do take the X96 Bus to Pireaus port, X96 bus is also good to take you from Pireaus to the airport. Tickets cost 5 euro and these busses run 24 hours. Taxis to the city center from the airport cost 35-55 euro and expect to be there in 40 minutes. The bus needs an hour or so.
If you arrive to Pireaus by boat, you could get the electric railway it is in the center of the port opposite the road near E9 gate. The port is lengthy and there are busses that run inside the port from each port gate to the other, you may use them they are free.If you arrive to Athens by train you will arrive at Larrissis station, there is a metro station just outside.