Well, first you want to get to La Noscea, Bentbranch is in the Black Shroud. Just follow the road down south until you hit Thanalan, then follow the road west all the way through Thanalan until you get to the ferry docks where you can take the ferry to Limsa Lominsa. If this is for the summer event quest, I am guessing you will want to go to Wineport in Western La Noscea. You can do the quest in the Black Shroud (for green ash/green swimsuit) if you haven't already though, at Hyrstmill or Quarrymill.
Maps to help: http://www.ffxivmaps.com/
Good luck! :)