Traveling by train for my specific itinerary?

Hi Melissa. 1. My suggestion would be not to buy a pass; it's almost always more expensive and more of a hassle. 2. Use tgv-europe.com for French tix, trenitalia.com for Italy. If you buy tix in advance,they will be way less than the cost of a rail pass. 3. When you buy tix, reservations are automatically included.
4. No. Buy point to point tix on above web sites 90 to 120 days in advance. For Rome to Paris, consider flying; it's fast and cheap. Or if you haven't booked your flight yet, just fly home from Rome. Enjoy your trip!

Bob is quite correct. To add to his reply: First, if you haven't yet booked your flights, fly "open jaw," into Paris and out of Rome. Even if it "costs more" than a simple round trip (and it may not), you save time and money by not backtracking. To find these flights, don't look for 2 one-ways; instead, use the "multi city" function on Kayak. Even with a pass, you have to buy passholder supplements on all-reserved French trains and reservations on all-reserved Italian trains. So, a pass doesn't save any hassle, and usually doesn't save any money either. Sure, you can take regional trains (the equivalent of LIRR or Metro North) just by flashing your pass. But, your routes include almost none of these trains, and even if they did, the fares for them are much cheaper than a pass day. Paris to Aix is a TGV (mandatory reservations, limited numbers of tickets available for pass holders, inexpensive with advance purchase but very pricey at the last minute). Book on http://en.voyages-sncf.com/en/, the new and improved French rail website. For ticket collection country, select France. You will get an e-mail with a PDF; print this out (it says you need A4 paper, but 8.5 x 11 works fine). Show your passport along with the ticket to the conductor, and you're set. Marseille to Milan will require multiple changes. You may not be able to buy the whole thing on one website; try the French site above, as well as the Italian one http://www.trenitalia.com/ (click on British flag on upper right to select English). Others here may know more about this route (France and Italian rail no longer play nice with each other, so the connections are often hard). continued..

For Milan to Venice, Venice to Florence, and Florence to Rome, use Trenitalia. Register first; you get an e-mail with a temporary password, which you must use to change the password immediately. You can get discounts if you book in advance. You print out the ticket and show it to the conductor, and you're set. You should also notify your credit card company that you are going to be making foreign purchases, or the transactions may be blocked. Note that the discount advance purchase tickets for both France and Italy are usually non-exchangeable and non-refundable. So, while they save a lot of money, be sure of your plans before buying, and double check dates (there are no refunds even if you just made an "innocent" mistake like a wrong date). For your Italian routes, the full fare tickets are not expensive, so you can wait until you get there. For Paris to Aix, tickets for tomorrow (i.e., last minute) are between €90 and €154 - second class. But for the randomly selected date of July 17, it's between €39 and €45 second class, and first class is as low as €46. For much more detail about booking on the Trenitalia website, look at Ron in Rome: http://tinyurl.com/c4t693b. And for more about European trains, you can't beat the Man in Seat 61. For instance, if you are not able to get an open-jaw flight, here's what he has to say about Rome to Paris night trains: http://tinyurl.com/bwx957v