You can go by train (the cheapeast way; and very fast too). But it is naturally easier to drive by shuttle. There is a Rome Shuttle Lomousine Service from the port Civitavecchia to Rome (or viceversa) for about € 110 / $ 132 for 4 peaple with luggages; 6 peaople with Mercedes Van € 145 / $174. For more information click on this lin: http://www.romeshuttlelimousine.com/CivitavecchiaE.htm
Best regards
Uwe