There are two ways to get to St. John from St. Thomas. You can take the ferry from Red Hook or the ferry from Charlotte Amalie. If the timing works out, the Charlotte Amalie ferry is the way to go!
The Red Hook ferry runs every hour and costs $5.00 (baggage is extra). The problem with Red Hook, however, is that it takes about 40 minutes to an hour by taxi from the airport. The taxi trip can also be pretty miserable (on all of my trips to Red Hook, the taxi driver ends up backing into another car). The ferry ride itself takes about 15 minutes. (And they don't allow snacks on the ferry, but I think beverages are allowed)
The Charlotte Amalie ferry runs every other hour and costs $12.00(?) (baggage is also extra), but it takes about 10 minutes to get to the dock from the airport, and the taxi ride is much more tolerable. The ferry ride takes about 40 minutes.
Once you arrive in Cruz Bay, all of the car rental outposts are within walking distance. I recommend leaving part of your party at the dock with the bags, and just walking to get your jeep. And yes, you really need to rent a jeep or some time of SUV in St. John.
After you pick up the car, you'll need to immediately find the gas station (there are only 2 on the island) as all of the cars come empty.
Remember to drive on the LEFT!
Pick up the rest of the party at the dock, and then head to your condo/house.
For dinner, check out Morgan's Mango. Thursday night is half price margaritas with live music! (Reservations are recommended).