If you have a US passport and you are entering by plane, you don't need a Tourist Visa to enter Venezuela. A Tourist Card will be issued by the airlines which allows you to stay in Venezuela for a non-renewable 90 day period.
For more information:
http://www.consuladovenezuelasfo.org/tourist_card.htm
There is a departure tax that you need to pay before leaving Venezuela (I think it was $19). In addition, there is also a small tax that you need to pay when leaving Margarita (about $2).
Margarita is a beautiful island. Enjoy!