You have to go to the Largo della Pace Service Center to get a CSP City Bus. The bus takes about 10 minutes to get to the Civitavecchia train station.
CSP City Buses going to the station run about every 20 minutes.
Tickets can be purchased in the Largo della Pace Service Center, at the pizzeria Mastro Titta (located near Largo della Pace and open from 10 AM to midnight), at the authorized Coffe-Bar inside the ADM Terminal, or at the News-Stand and the Coffe-Bar inside Civitavecchia railway station.
One way ticket costs 2 Euro (luggage included).