I’d like to advise you how to spend one day in Rome to learn about it long enough. It is my personal guide which I tested during the Italy vacations. Actually one day is not enough to see all Rome attractions. To see all Rome attractions you need a time to get around this great city, and a walk in the narrow streets of Rome with other tourist.
First of all take the metro to the station «Ottaviano-S. Pietro» and go to Vatican, in the direction of St. Peter's Square (Piazza S. Pietro). You will walk along Via Ottaviano and be sure that now you're in the Vatican, in a most unusual state of the planet.
Then down from the dome, to see St. Peter's Basilica itself. If you are lucky, you can be in the St Peter's Basilica during the divine service. Or look at the divine service from the first tier of the dome under the road to the observation deck.
Remember you can not enter into Basilica with a knife in your pocket, in shorts and T-shirts.
To make the most of your visit to Rome go on a perfectly-planned route of Rome by Sightseeing bus tours. As always I advise you to sit on the top floor of a tourist bus, put on headphones and see Rome at your own pace. Bus stops near Rome main attractions and you can spend as much time as you like at each attraction in Rome and then take the next Sightseeing bus tour for guided tours and city sightseeing Rome. In the bus there is coupon booklet for discounts to museums and some attractions .
Of course I advise you go to the pizzeria, buy a pizza or pasta take-out and go to one of the parks and squares of Rome, to sit on the grass, looking at the map of Rome, read the guide and enjoy life.