Public transport is limited; your best bet for seeing the island is to hire a car.
Budget (647 1446), Hertz (646 5875), National (649 0110), Toyota (642 3200), Avis (646 8156) and Nissan (632 7300) all have rental booths at the airport. Optional collision damage waiver (CDW) is available and most companies will deliver cars to your hotel at no extra cost. Tamuning Plaza Hotel operates a very reasonable car-hire service; the Harmon Loop Hotel has good deals, too.
Guam Mass Transit Authority (475 4682) operates a limited public bus system. The most useful route is from Hagåtña to Tamuning, Tumon Bay and the Micronesia Mall; another line runs between Hagåtña and Agat; and another between Hagåtña and the University of Guam.
Buses run approximately every hour from 5.30am to 7.30pm from Monday to Saturday (US$1/3 per ride/day), although frequencies can be erratic and inconsistent.
Bicycling is dangerous and not recommended - on Guam's main roads there's hardly any room for pedestrians, let alone bicycles.