Termini station is pretty accessible in Rome as Rome is a very walkable city. That’s the station you’re likely to take a train to civitavecchia. I will forewarn you, be very very careful of pick pockets and thieves there. A purse was grabbed out of a woman’s hand in broad daylight by a man on foot. We like staying in the piazza where the pantheon is. There’s a lot of people watching there and it’s a short walk north to Vatican City. Don’t eat in squares. Ask a hotel person to send you to a traditional family restaurant on the back streets. You won’t be sorry.