I can't figure out which bus to take to the various places.
Which bus should I take to get to the St. Peter's Basillica from the Spagna stop? Should I take a taxi?
Which bus should I take from Ponte Santa Angelo to Spagna?
From Spagna to the Palantine Hill/Forum? From the Colleseom to Camo di Fiori? Can I walk from Campo di Fiori to the Pantheon?
Can I take a bus from Piazza Novona to Spagna?

Go to http://www.atac.roma.it/page.asp?p=2.
On the bottom right open up Roma centro.
If you print out some pages (make them big enough to see) ,you can gets answers to your questions. You can also buy a map when you arrive that .
Instead of Spagna, go to the St. Peter's stop.There's a number of buses (or walk)from there.
You can easily walk from Fiori to the Pantheon.

As soon as you get to Rome, buy the so-called Metrobus map - available at all newspaper stands, which can be found at almost every street corner -, which shows all the bus routes and stops.

http://www.fodors.com/forums/threads...p;tid=34908741

Go the atac url given in jabez's post.
Hit the calcola il percosa button on the left.
On the new screen, hit the english button.
On the new screen, type in your address and your destination.
This gives you the bus or metro to take.