We don't use buses at home in England and, in London, I always recommend the underground but, in Rome, we found the buses to be cheap, fast and convenient.
Tickets can be purchased in most tobacconist shops - we used the all-day ticket which is pretty cheap and, as the name suggests, lets you use the buses all day.
You must remember to feed your ticket through the little yellow machine on the bus the first time you use it as a heavy fine can be levied if you don't - Not that anyone ever checked our tickets!
If you don't know your way around Rome, arm yourself with a good street map and a bus route map (the route map is also a street map but it's not a very good one!)

