Both Trunk Bay and Cinnamon Bay are beautiful beaches but as Dan said, make sure you get back to the ship in time as traffic can be really heavy. Also you will have to check out the times the ferry is running.
If it were me and my first time in St. Thomas, I would take a ship's sponsered tour of the island or there are lots of taxis waiting at the pier to take you on a tour.
You might also want to go to Megan's bay and spend the day there. It's a beauitful beach also. This is what we often did in the morning, then came back to the ship for lunch and then out again for a little shopping.
Being a first time cruiser I personally think you are taking a chance going all the way to St. John's. There really is nothing to see there - just their lovely beaches and the expensive homes/hotels.
There are many videos on You Tube of passengers, who look like they have been at the beach for the day, running down the dock as the ship leaves the port.......only in very rare instances will the ship's Captain back up the ship for these latecomers. Not a nice sight or feeling.
Please, I'm not trying to put you off going to St. John's, but think twice about it.
Have a great time & enjoy whatever you decide to do. I hope you will love the Eurodam - we will be on her for our 3rd time at Christmas.