Once you're inside Rome Termini, you'll have to keep an eye on the departures board and about half an hour before your train to Milan is scheduled to leave the platform will be announced. Pretty straightforward.
When I read you posts though, the real issue from my perspective is your strategy on how you plan on getting from Civitavecchia port to Termini and the timing... you ought not shave things too close on your train from Ro e to Milan. I suggest the earliest you plan on doing that is 1050 and 1150 would be better.
If you are OK to walk with your luggage, pick it up in the hall upon arrival and take the shuttle towards Largo della Pace. Get off at the Michalengelo Fort and walk 15 minutes to the station. There are sidewalks and it is a straight shot on level ground.