It’s easy to buy tickets in the train station.  You can buy them at the automated kiosks (very user friendly in English) to avoid waiting in-line.  If you choose a specific train that requires a reservation, it will be included with the ticket.  You can also buy them on-line at www.trenitalia.com  You get a confirmation number and then you can print out your ticket at the kiosk or, on some routes you can go “ticketless” with just your home-printed confirmation.  Don’t buy on-line tickets from any other source as they will cost more, although you can sometimes find good prices at www.italiarail.com 

Personally, I’d wait until you get to Italy since your plans may change.  However you can usually change the reservation one time (before the original departure date) and use your ticket on a different train at the kiosks.  Sign up at trenitalia.com and get your ID and password before you leave home in case you end up needing to buy a ticket on-line

Agree with oldlady, buy tickets there.  But I’d add that you should check the trenitalia site to have a look at prices and routes before you go – make some tentative plans.  Italian trains can be a little weird.  If you’re going Siena to Rome, for example, and you just walk up to a counter to buy a ticket, the system will often route you through Florence, as that’s often the fastest.  But a more direct route that costs you a few more minutes is half the price, as prices are based on kilometers traveles.  I found a similar situation going Rome to Pisa.  So play around with the options a bit to figure the best routes, departure times, and prices, and take this info with you.

Also, you’re travel some hefty distances.  It could be that an Italian rail pass is more economical.  Use trenitalia to add up your travel costs and compare to the pass price.  Buying this pass before you leave is usually the cheapest option.

Kingsolomon — check prices on every train (you can do it at the kiosks).  EurostarItalia trains often cost twice as much as the regular fare (due to the expensive reservation) and on short trips are only a few minutes faster than cheaper intercity trains and much cheaper regional trains.  

thanks alot oldlady and russ…good info im definitely going to use before i got
 
sal

I’ve been reading a lot of these questions & I think oldlady deserves a tip (actually I think we need to put her on a salary!) her answers are all very thought out & precise. It’s like askjeeves only smarter.
keep up the good work