First you buy a ticket from a counter or a tobacco store, go on the bus, then validate the ticket yourself. I bought all my tickets until I noticed that the locals didn't really validate theirs. At first I thought they were holding some season ticket. But later I found out I was wrong. Most Italians I saw don't pay for their fares.
Most tourist do.
Anyhow, I had no problems for the next 12 days or so. Walking around, no doubt you will get thirsty. Bern quenched her thirst courtesy of the wonderful street taps that the Italian government has placed all around Rome. The humble street tap is a novel design with 2 holes whereby if you stopper the lower one with your finger, water sprays out through the upper one for some quick thirst quenching action. Not many temples can match St Peter's in terms of historical significance or architectural splendour.