Laura-
In St. Marteen, there are several rental car "booths" to the right as you exit the pier. At the end of the pier there is a sort of welcome center. To the right of it is a covered walkway area. AT the end of the walkway you will find the rental car booths. Don't worry about booking in advance. There will be several "salesmen" in the walkway area who will have photos and rates for plenty of cars. Don't say yes to the firsr, bargin with a couple for a good rate. We rented a Suzuki 4x4 for about $50.
In St. Thomas, there is a BUdget (I believe) office close to the pier. Next to the pier, there is a large shopping mall. Several car rental places are scattered through it. However, I know for sure there is one across the street from the mall. It is to the left of the big "beverage" outlet that you can see from the pier. You'll know what I mean when you see it.
Couple of tips for you....St. Marteen driving is just like USA, on the right with left hand drive. In St. Thomas, you get left drive cars and drive on the left. If you've never driven on the left, be careful.
DOnnie