Map of Sounion Greece
The easiest way to get to Cape Sounion Greece is to take one of the organised tours. You can book these through your hotel (usually) or directly from a travel company. Some of the tours leave at about 2.00pm, returning four hours later. Unfortunately, this normally means you don't see the sunsets for which the site is renowned. It also means you have a limited time at the site itself, which might be all right for a casual visit, but not long enough if you want to really explore.
You can also take a bus which runs on the senic coastal route (be careful, as some buses use an inland route). These leave Athens hourly from the bus terminal at Mavromateon.
If there are enough of you in your group you might be able to negotiate a reasonable price with a minibus company or similar. Alternatively, check out bus timetables to see if any of them would be suitable.
To visit Thorikos probably the best way is to hire a car. In fact, if you plan to spend time at Sounion, Thorikos and Lavrion, a car would allow you to do as you wish. Just remember driving in Athens is no joke.
If you are thinking of taking a car and make a day of it, there are quite a few other ancient Greek remains in Thorikos and Lavrion. Thorikos has one of the earliest Greek theatres, first set out in the 6C BC. There is a washery and mine gallery used as part of the silver mines, and you can also see some Mycenaean tombs which have been excavated. Lavrion has some washing cisterns, workshops, and other remains from the 4C BC.
Map of the area
This map shows you where Sunion, Thorikos and Lavrion are in relation to Athens (just hover your mouse over the red markers). You can use the controls on the left to zoom in and out of the map, and you can move the map around by holding down your mouse button and moving the mouse around. The "Satellite" view isn't especially interesting as the resolution for the area isn't as detailed as that for Athens itself.