As others have said the process for finding and boarding a train is pretty straightforward. There are signboards with pending trains and trains currently boarding with track numbers and times like an airport. If you are heading to the final destination of a train the signboard is easy to read. If you're getting off in the middle - like getting off at Orvieto from the Rome train - you should check the train number to find the right train. You can also find this information in the Trenitalia app which is handy if you are changing trains and want to know which track to head to when you get off the first train.
Fast trains have assigned seats and many train stations will also sign which car is likely to stop where so you can be in the right place for car "C" or whichever one you have. The train cars themselves will tell you which door is best for which seats - 1-13 this door 14-26 the next door - to minimize walking inside the train. Really modern fast trains even have LED displays on their doors that indicate the train number, car, seat numbers etc.
Most Italian train stations have the tracks laid out next to each other numbered 1-22 or whatever the process of catching or changing trains is essentially simply walking down to the track number and then down to the train. The only big exception to this I have found is the Bologna train station which goes down several stories deep and requires a series of escalators. I mention this because it is the only Italian train station I've seen like this after having been through most of the major ones.
It's easier than you're thinking. Remember that 1,000s of Italians do this everyday and go with it.
Have a great trip,
=Tod
