I would start at a beach, not sure it matters which area, except as dcassetta pointed out, sometimes the waves/currents can be strong at different ones. If there is a beach with a cove, that is best.
St Thomas and St Maarten, the actual snorkeling excursions (not the beach ones) take you to sites where there is no beach. You hop of the edge of the boat and climb back up the stairs of the boat. St Thomas has been my fave snorkeling spot so far, but I was not near a beach.
If you can, I would also try to arrange a private tour. For new snorkelers, it is a bit hard to begin with as the breathing process is not natural and then, you have many swimmers who forget that they have an additional 6-12 inches on the ends of their legs due to the fins. Nothing like swimming along and getting kicked with a fin. And this is where calmer/less people/and ability to stand up relatively soon come into play.
Also, practice in a swimming pool before hand if you can. I would recommend purchasing your own snorkel gear if you think this is something you will enjoy long term, having a water cut off valve on the snorkel is awesome, it basically shuts off the end of the snorkel if it gets water in it. You don't breathe in water this way.