I stayed at a villa overlooking fish and reef bays. There is a small parking area in front of one of the villa's. This is not the start of the Reef Bay Trail, rather near the end. Short walk to the beach then follow the coast line to the sugar mill ruins. The walk along the beach is fine with some scrambling over rocks. Never went to the start of the trail so cannot help you there.

john

Hello Greg,

There is off the side of the road parking; not a parking lot.

--Islander

The'official' start of Reef Bay trail is along Center Road and there is very limited parking for about 5 cars. The problem I find is the 'parking' location comes up so fast you run the risk of flying past it and then looking for a place to turn around. If you intend to park there, it implies you are hiking back up the trail to get the car so I would plan an early trip, get the spot and take advantage of the cooler morning. Unless you are not into the group thing, you might want to consider the Parks tour which takes you to the start, is led by a ranger (complete with descriptions, explanations, etc.) and takes you by boat back to Cruz rather than hike back up the hill.
We stayed in Reef Bay area and although you can park along the road next to the trail accesssing the beach, it is not a parking lot and was being used by local workers repairing a home. The trail begins (no sign) just South of the wood shingled house in Reef Bay. As John mentioned, you then follow the coast East until you reach the trail spur (again, no sign). I suggest sneakers over sandels since the rocks you must scramble over do require better footing. There are several places were trees, etc. break the coast and require you to find a path around them back to the beach. Not a problem so don't let that dissuade you.
Once in the Genti Bay area, you will come to an outcropping that appears too involved to climb over. It is unless that is your pashion so turn back about 20 yards and look for the trail spur. It goes back about 10 yards and then takes a right straight uphill parallel to the shoreline. Follow this back down to the Reef Bay trail at which you will finally see a sign directing you to the Sugar Mill Ruins (right) or Petroglyphs (left).
We took the Reef Bay to Genti hike, saw the Petroglyphs, Sugar Mill and snorkeled in Genti. Started at 9 AM and were back at 1 PM.
The book "St. John Off the Beaten Track" is a great reference.