Cetti Bay is on Route 4 (Marine Corps Drive / Marine Drive), which follows the southwestern coastline of Guam. From Tumon Bay (the main resort area in the north), take Route 1 south through Hagatña and Agat — total driving time is around 45 minutes to an hour depending on traffic and stops. The Cetti Bay Overlook has a small car park on the roadside; it is hard to miss.
There is no public transport to Cetti Bay. A rental car is the practical option for the day. Car hire is widely available in the Tumon and Hagatña areas. The road is sealed all the way and accessible in a standard car, though some of the side tracks and trailheads are better suited to a 4WD.
Ride-sharing apps and taxis exist on Guam but are not well suited to a full day on the southern coast. If you plan to hike Mount Lamlam and visit Fouha Point in the same day, a rental car gives you the flexibility to move between trailheads without back-tracking.