getting there: metro - linea A/Repubblica or Barberini
open: the church has been closed since 2013 due to extensive damage to the ceiling inside
I found walking Rome was the best way to see everything.
The metro, trams and buses are also an easy and cheap option.
Buses and the metro can get crowded. Tickets must be bought before boarding and validated.
Beware of pickpockets.
Buses 40 (express) and 64 start at Termini and end near Saint Peter's, traveling past places of interest, returning the same way.
Some stops along the 64 route are:
Corso Vittorio Emanuele II
Bus 75 takes you past the Colosseum to Trastevere
Bus 910 takes you to Villa Borghese
€1.50 B.I.T (Biglietto Integrato a Tempo) is the standard ticket valid for one metro, unlimited tram or bus rides within 100 minutes.
€7.00 B.I.G ( Biglietto Integrato a Giornaliero) is a daily ticket valid for unlimited metro, tram, bus and train travel within Rome.
Purchasing Bus and Metro tickets.
Newsstands, train stations, metro stations, kiosks with the ATAC logo and tabacchi shops sell tickets for the metro, trams and buses.
Large fines apply to travelers not holding or validating their ticket. Tickets once validated start from the time they have been stamped.