Rome to Civitavecchio - easiest way for single senior

You could book a private driver in advance, but it will be quite expensive. Or take a cab from wherever you are in Rome, I think the fixed fare is 120 €.

Can you contact other passengers online and share the ride?

Sorry its a long thread, but you'll find a lot of info here:

https://community.ricksteves.com/travel-forum/italy/rome-airport-to-port-outside-of-rome

Assuming that you are going to Civitavecchia to embark a cruise ship, I'd recommend looking on Cruise Critic. Find the subforum for Roll Calls, then go to the Roll Call for your specific sailing. You may be able to share a ride with fellow pax as Dario suggested.

This is who you want. Awesome service. we used them for an airport pick up and to the cruise port.
https://www.romecabs.com/transferservice/airports/