Rome's metro is laid out in a north-south, east-west cross, and they intersect at Termini Station. They're quite handy if you're staying near a stop, but if not... well, get to know the buses.
The system can seem complex at first, but routes are laid out in an easy-to-understand manner at stops. Buses are frequent, but subject to the low-level chaos that is Roman traffic!
Transit tickets are valid across all modes of public transport and readily available at vending machines at metro stations, major bus stops and at some bars and tobacconists. Don't forget to validate them after boarding the bus!