Once you arrive at Civitavecchia train station you can take one of the CSP (Civitavecchia Public Services) or PortLink buses or walk to the port.
The CSP buses leave from the square in front of the station every 20 minutes and lead you inside the port in 15 minutes (approx). The ticket costs € 2.00 (luggage included) and can be purchased at the newsstand and the station bar.
Bear in mind: buses from the station will take you to Largo della Pace, this is the main hub of Civitavecchia port for cruises. From here you can take one of the free shuttles that depart frequently and that will take you to your terminal or to the dock of departure.