I don't know anything about how you travel, so I can't say how far in advance would be best for you.
Once you have your reservations you then know exactly the train number, its departure time from where you will board it, the car number (carozza), and the seat numbers (posti).
When you go into the station you look at the big board or the departure screen - just like in an airport - that says Partenza (departures). Find your train number and if you are not too early it will show what platform (track)(binario)(bin) your train will be on.
Walk to that track. In a small station you may be standing on it already, in a big station it may take you 5 or 10 minutes walking, in a very big one a bit longer, depending on how fast you walk.
Each car will have a clear indication of its number on the outside as it arrives. If it is a major train you may be able to see a printed display in the case on the platform which shows how the train is laid out and where your car is. Usually they are in ordern with the smallest number either at the front or rear.
Reading all that you should be able to come up with a good guess of how much time you will likely need given your circumstances.
If the train originates at the station it may be there several minutes prior to departure, and if a dead end station like Roma Termini likely 15 or 20.
If it is an intermediate station the train may only stop a short while and you will need to hop on.
On the Circumvesuviana the train will stop at intermediate stations for only a few seconds, but there are no reservations or passes on that train.