To add the the great advice you've already gotten, that a rail pass is not a good idea for this itinerary:
A great source for information about European train travel is The Man In Seat 61. Here is his discussion of whether you should buy a rail pass. Read it thoroughly and make sure you understand EVERYTHING in it before making any decisions: http://tinyurl.com/bkw4u6c
For domestic trains within Italy, and for international trains from Italy to Switzerland and Switzerland to Germany, if you can book early online and can commit to non-refundable and non-exchangeable tickets, you can save a lot of money. For domestic tickets within Switzerland, there is no discount for early booking.
Next, to get the real prices of trains, you need to go the the website of the company that operates the train (as said above). For Italy, that's usually Trenitalia: http://www.trenitalia.com/tcom-en
It often helps, even on the English version of the site, to use the Italian names of the stations:
Rome - Roma Termini
Venice - Venezia SL
Naples - Napoli C
As Nigel said, you are backtracking, and should see the Cinque Terre between Rome and Venice. In order to get schedules, you will need to put in a specific one of the five towns; you can use Vernazza just to get an idea.
From Sorrento to Rome, you take the Circumvesuviana to Naples, then a Trenitalia train from Naples to Rome. The Circumvesuviana is cheap and has no reservations (think of it as a "bus on wheels"). It also accepts no rail passes of any kind. The ride from Sorrento to Naples is about 70 minutes.
On fast trains in Italy (most of the ones you are taking), there are three classes of fares. Super Economy is non-refundable and non-exchangable; Economy can be exchanged, but not refunded; and Base is full fare. If you want Base fares, or if these are the only ones left for your dates, there's no advantage to buying in advance. Each fast train ticket is only good for ONE specific train; if you get on ANY other train, you'll pay a heft fine. So, if you miss the 8 AM from Naples to Rome, you can't just get on the 9 AM (even with a Base fare ticket); you have to change your ticket first. However, if you're using a pass, the same is true of reservations; if you buy a reservation for the 8 AM train, it's only good on that train. So, having a rail pass does not add flexibility, since you can't just hop on a fast train. You can hop on regional trains, but these are cheap, and not worth using a pass day for.
For Switzerland (and Switzerland to Munich), use the Swiss Rail site. Note that the default price you will see on this site is with a Half-Fare card, so if you're not getting one, make sure you are looking at the correct price: http://www.sbb.ch/en/home.html. Swiss trains are more expensive, so a Half Fare card or some form of Swiss Pass can be a good deal.
Most people find second class fine, but some do prefer first class. If you do want to ride first class, the price difference between a pass and point-to-point tickets is lessened. On the other hand, for Italy, if you can book early enough and can commit to non-refundable and non-exchangeable tickets, you can sometimes get first class tickets for little more than second class - a win-win.