Carrie,
I'm kind of going to make a leap of faith here and assume you mean San Juan, Puerto Rico and not St Juan. (Unless there's an island called St Juan that I never heard of and in that case I apologize! 🙂
In order to get to St John from San Juan, you will need to fly to St Thomas and then take a ferry to St John. There are several airlines that make the trip. I would suggest going to Orbitz or Expedia and getting some prices.
There is a ferry from Puerto Rico to St Thomas but it only runs a couple of times a month and it's a loooooooong ride in open water.
Once you're on St Thomas, it's a short (less than 30 minutes) ferry ride to St John!

American Airlines should have connecting flights to St. Thomas. There are several other commuter flights. You could try using the USVI yellow pages at http://www.usviyellowpages.org and look up airlines. The ads are there, so you can see which ones go from San Juan to St. Thomas. The ferry schedules are available on this site for your reference. Go to the home page www.vinow.com and there is a link in the frequently asked questions for ferry schdules.

--Islander

When we went 11/2002, we took American Airlines. They had nonstops to St Thomas from San Juan, Miami, Orlando, and Boston. Our's went DFW-MIA-STT. It is just over 1000 miles from Miami - about three hours. (We knew of all the nonstops because we were hoping to get bumped on the way home, and sure enough, almost did... and they looked up all the nonstops out of STT just in case for us.)

Note: if you happen to be using American AAdvantage miles, it is about 30k per ticket. Also, you can do the 'stopover' in San Juan -not something they advertise, but we learned about it by accident. We had a cruise out of San Juan in 2001, but wanted to visit St Croix for a few nights after the cruise. I was able to use a 'stopover' in San Juan for those 8 nights, then pick up on my ticket to St Croix. Stayed there 3 nights, then returned back to Dallas. All one ticket - and just the 30,000 miles. Stopover is outbound only, not the return trip.

🙂

Also, it is very easy to get to St John from STT airport. We landed at 3:11, waited for luggage, caught a taxi (readily available at airport), and made the 4:00 ferry from Charlotte Amalie (last ferry of day). 45-minute ferry ride for $7 each directly to Cruz Bay. Very convenient.

-Mandy in Dallas