Rome is definitely one of the best cities to see by foot or bike. Most sites are within walking distance from most hotels situated throughout the inner city.  One of the best ways to get a lay of the land is to get a local map of the city - notating all of the major sites (most hotels can supply you with a city map).  Get started by just walking and observing when you first arrive - just look around and be observant - see all of the splendor that is around you - from the very large to the little hidden gems scattered throughout the city.  Be adventureous and not afraid to get lost as walking is truely the best way to see this city.  (And do not forget a pair of very comfortable walking shoes.)  If one also has the opportunity to go about the late evening after the sun goes down and the city quiets down (the later the better) this can be a time to see Rome with a very unique flavor.  An evening drive throughout the city while the lights of the street reflect against the many ancient sites - you will be awestruck by what you are seeing - as if you are dreaming - but alas you are not dreaming you are in Rome!