There's a bus every 5 minutes or so, which will get you in the hearth of the Histotical Center of Rome in 5 minutes.
As usual, get standard tickets in advance at tobaco shop or news paper stands, or get a Roma Pass.
Most of time, the 100 minutes ticket at 1,5 Euro will be enough.
On the map, main bus stops:
<ul>
<li>Via Viterbo, from B&B Savoia to the archeological area
<li>Tritone, to Fontana di Trevi, Piazza di Spagna, piazza Navona
<li>Piazza Venezia, to Colosseo, Campo de' Fiori, Ghetto
</ul>