Sounio is in Attica.
A bus from Athens costs €8. The buses start from a terminal at Κότσικα 2, Αθήνα 104 34, near the southeast corner of the Areos Park a few blocks east from the Victoria Metro station (but there are more bus terminals in the area - ask in the KTEL kiosk). This is a short distance from the Archaeological Museum, so you might like to combine your visits. There is one bus per hour during a day. Buses also stop on some other places in Athens. The terminus is just below the temple. The last bus back to Athens starts at 20:00 in Sounio, late enough for the famous sunset over the temple. One way takes aprox. two hours.
Sunset over the temple. The temple area closes just after the sunset.
As well as hosting the temple, the cape was a fortified port in ancient times. The entry ticket lets you explore the full site - note that the ruins are quite modest, but the whole site is very scenic and there are information boards (with English) to help you imagine how it used to look. Keep an eye out for the Chukar partridges that live on the cape. To make a full day out of your visit, you could relax at one of the nearby beaches.
There is a taverna just below the temple, next to the bus stop. It is rather expensive and not very impressive.