You can book train tickets here on http://www.trenitalia.com/
Simply, switch to English (clicking the British flag in the top right hand corner of the screen)
Please populate the FROM and TO and approximate times to do a search on what is available.
Main Train Stations:
Rome: Roma Termini
Milan: Milano Centrale
Venice: Venezia S. Lucia
Florence: Firenze S. M. Novella
Naples: Napoli Centrale
Como: Como Nord Lago or Como S. Giovanni or Como Nord Camerlata or Como Nord Borghi
You’ll then be able to see the various trip options. Remember to tick the “unregistered user box” so you won’t have to create an account. The sooner you book, the better since cheaper tickets will sell out first.
If your credit card is declined, please contact your bank because it most likely is blocking the charge due to suspected international fraud.