What is the easiest way to travel from Florence to citavecchia where we will be getting a cruise ship
Train. The train station in Civit..... is a ten minute walk along the beach to the entrance of the port. From there a shuttle bus will take you to your ship. Probably the best route is Florence to Rome to Civit... There are trains from Florence that change somewhere else. No direct trains to Civit.. Use trenitalia.com to check schedules.
There is also a train route through Livorno and Pisa, but probably through Rome is the fastest.
Having done this I vote for Florence/Rome(Termini)/Citavecchia. At the port exit the train station, walk through the parking lot towards the ocean, turn right along the main street (Viale Garibaldi) fronting the water (this is a short distance - 1/2 block maybe - I forget). Where the street turns a hard right and uphill after a few short blocks you will see the port entrance on the ocean side of the uphill turn. Port shuttles (free) will take you to your ship. There's a good U-tube video on this if I recall correctly on www.romewalks.com Anyway I always enjoy the train ride from Florence to Rome and the one from Rome to Citavecchia. Just give yourself lots of time for this and treat it as a great site seeing day. Don't stress yourself out trying to make tight train connections. There are lots of trains (fewer on Sunday).
I am not sure of Marie's comment about a hard right and uphill. Once you cross the train station's parking area, down the steps to the street below, cross the street to the beach and turn right. It is a wide sidewalk that follows the beach. It is a straight shot to the port entrance - something between a quarter and half mile. When you exit the train station and look a little to your right you should see your ship. Just follow the flow, lots of people will be headed in that direction.