There is a bus from Rome to the entrance to the old town. The Rome terminus is Lepanta Metro station on Linea A. There is also a train but the station is less convenient.
If you hire a car at Rome's Fiumicino airport follow the road to Rome for a few km and then take the Autostrada signposted in the direction of Civitavecchia. The exit to Cerveteri is about 15 minutes later, depending on how fast you drive. The Necropolis is to the north of the modern town and reasonably well signposted.
There's a local bus from where the Rome buses start to the necropolis. The old town is very small and walking is all that is required. The new town lacks interest.
The museum in the old town is also well worth a visit.
The Bed and Breakfast Casa di Anna at Largo Boccetta 5-6 is very good and, equally important, seems to be the only accommodation in the old town.