As Frank said, there's probably no benefit to purchasing tickets in advance at this late date: economy or super-economy tickets are probably long gone.
In your case, I'd go to a ticket window at the station, tell the attendant where you want to go and when, and just buy them there. I advise 2nd class tickets on faster trains with reserved seats as there isn't a big enough difference between those carriages and first class to warrant any big difference in price.
He/she will not have time to answer a lot of questions - such as which platform, which carriage, how find assigned seats, etc. - so you'll want to familiarize yourself with how to read departure boards at the station and reference the "Man in Seat 61" website pages on Italian trains. They are not difficult once you get the hang of them.
A side question? Apologies if I'm off base but is it possible that English is not your first language? I sense that it may not be, thus some communication difficulties.