How do you know where to go for a quest?

[NemoNobody]

I took the six day stilt quest in Joppa but I'm not clear on where I need to go

[RozetaMeteor]

try talk with people around the town, specially the warden, she might give you some clues

[Pixel Peeper]

The "look" function is usable on the world map. You can check out the names of the various main settlements, including the Stilt (which happens to be in the northwestern corner of the map).

[glass zebra]

"Journey through the Great Salt Desert to visit the merchant bazaar and Mechanimist cathedral, where a proselyte asked you to make an offering of a trinket." should be written in your journal.

The Great Salt Desert is that area that covers the western edge of the map and the place you have to visit is the only non dune tile in the desert to the north.