These are easy, get the quest from their hut/shack on an outpost. They are located in dark rooms at the edges of the outposts and look like gypsies.
Typically the starter quests are going to be to go to an island and find the captain and bring back his scull. This translates to, head to the island and kill the skeleton spawns until the Captain skeleton spawns. His name will be in RED text, take him down and he will drop the skull. Pick it up and go for the second one. In my experience you usually have to collect 2 skulls on 2 different islands to complete a voyage.
Order of Souls shacks are in different locations on all the outposts. Their rooms are typically darker with glowing orbs (crystal balls) and they wear robes. Most of the ones I've seen have black all around their eyes.
Outpost Locations: (map grid)
Ancient Spire Outpost V-21
Dagger Tooth Outpost Q-8
Galleon's Grave Outpost X-9
Golden Sands Outpost E-12
Plunder Outpost M-22
Sanctuary Outpost G-7
Hope this helps....really if you just play and try everything you'll get it easy peasy.