In short: Catch a thief. Start at the inn after completing several of the initial quests.
Difficulty level: Easy. The quest doesn't require levelling up - just a certain level of dexterity.
After a conversation with Pablos, go to a nearby shop and talk to Aestella (the shop is open only during the day).
Then, find Heraldo (he's in the fishermen's hut in the morning). Go back to Pablos. Tell him you're ready to set the trap.
Patrol the streets until you see a fleeing thief - he's usually around the main street.
The best way to catch him is to drive him towards the beach and wait at the crossroads (between the inn and the shop). When he turns back towards you, run and seize him.
If you're having trouble catching him, throw in a few punches when he'd passing by. It'll stop him for a moment.
Finally, report back to Pablos.