Mily,
My first suggestion would be to pick up a copy of the Guidebook Europe Through The Back Door, as that will answer your questions on things like the train system.
Also check your local Library or Book Stores for country or city specific Guidebooks. Those will provide information on cities and sights in each city that you may want to see. It's difficult to offer advice on "the best parts (cities) to see in Italy" without having some idea of your interests and what you want to see.
Another good source of information would be to have a look at some of Rick's PBS shows on Italy and England, as they'll provide lots of visual information to help you decide.
It would also help to know the duration of your trip. That will have a bearing on how many destinations you'll be able to fit into the time frame you have. Summer is not the best time to travel in Europe. It's often hot and crowded and especially in Italy, the locals take their holidays in August so some places may be closed. I'm assuming you're planning to use open-jaw flights, inbound to London and outbound from Rome?
It will be easier to provide information on the trains once your Itinerary is somewhat definite. You many find it helpful to download the free PDF Rail Guide from this website. Click the "Railpasses" tab at the top, and then look in the lower right corner. In some cases a Railpass is a good idea, and in others buying P-P tickets is the most cost effective option. There are a HUGE variety in Railpasses, so it's a good idea to do some homework on that.
Is there a possibility you might make a stop in Switzerland on the way from Paris to Venice? That would make a good intermediate location to "break up" the rail trip.
Good luck with your planning!