As said before there are a few boat services from Trinidad to Tucupita I wrote them in the thread mentioned above, at the end.
But it depends on how much you will want to spent. If you alrrive in Trinidad(Port of Spain) you have flights with caribbean , venezolana, rutaca and turpial airlines, to Caracas, And from there you can take a non expensive flight to Margarita(better buy this one in advance using an agency). It would be more practical than the boats to tucupita, and with the Venezuelan airlines it's not very expensive(but not easy to buy, you have to call them or try an agency).
The other option is to go to Miami, and from there to Caracas.
I suppose that you now how to arrive from Barbados to Trinidad or Miami( liat, caribbean etc.,)
good fortune tours barbados