There'll be specific NPCs that offer quests at 13 to unlock the other souls. Just look for the green circles with exclamation marks.
Also, just hit 20, and quite enjoying things too. Wound up pugging Iron Tomb dungeon as the healer, and it was pretty good. Some healing intense moments in there. Almost wiped on the last boss because I was having some LoS issue that wouldn't let me heal a bunch of they group when we ran to Eliam for his protection. Wound up being just me and one warrior for the last half of the fight, but we won.
I've really gotta figure out what people are doing to get such crazy healing numbers in Warfronts. At lvl 19 I managed to heal for 37k in one match, and thought I was doing well until I saw others there that healed for almost 50k. I wonder if they're using a different build for heals. I'm going mostly sentinel with some warden for HoTs, and a bit of purifier just to get the shields.
In any case, fun game so far. Hopefully I'll hit level cap in a week or two and can start checking out end game.