Stonehenge is in the middle of no-where, right off of the main road. There is parking at the site where you follow a path leading under the road and then around the stones themselves (they do not allow you closer than about 4 meters to the stones due to the damage to the surrounding soil from many years of tourists).
An audio guide is available on the site for a small fee though you can also catch a guided tour (and a bus ride to boot – seeing other local sites) from the local town of Salisbury.