The first thing you'll want to do when you arrive at the Impetusium is grab the quests from the NPC's at the ground level camp. They are on the western side of the large structure in the middle.
Once you have all of the quests you will want to use your wings and fly up to our camp up top here. You will want to accept the quests up here as well and also set your Obelisk point here. You should have the following quests:
The first quest you will want to do is "Rebuilding the Pillar". This quest requires you to collect the building debris from all around the Impetusium. They can most commonly be found on the northern side of the Impetusium. Once you're done that quest circle around to the southern side of the Impetusium and kill 3 Mist Spirits. The Mist Spirits can most commonly be found in the waters surrounding the pillars.
After you've killed all 3 Mist Spirits turn both of these quests in (One is turned in up top and the other down below). Accept the follow up quest from Gark whom is up top. After you've done that head back down below.
NOTE: If your gathering is high enough to gather Blicora (85) feel free to complete the "Disgruntled Workers" quest. Also the Campaign quest to kill 3 Hero Spirits will open up another quest from Andgar that is worth completing. However, the second part of that campaign quests chain requires you to collect 3 pieces of Umkata's shit. Which isn't worth doing.
Down below you will want to do "Turbulent Splash Spirits" , "Soul Essence" and "[Coin] Mutated Spirits". The Splash Spirits can be found in the same area the Mist Spirits where found. Also the ghosts for the "Soul Essence" quest can be found just about anywhere. As for the [Coin] Mutated Spirits quest you will want to grind this until level 19. The quest doesn't give good XP but it will give you Iron Coins which you can turn in for gear or weapons.
Another option from 18-19 is to do the Blackclaw quests or more commonly known as "BC" quests. These quests include the following:
Either way, once you reach level 19 you will want to return to Altgard (Quickest way is by using the scroll in your inventory, if you saved it). Once you're back in Altgard you will want to take the Teleporter NPC to Brusthonin.