If Initiative was rolled, turns proceed in order according to the Initiative values. The entity with the highest result goes first. Players win ties for Initiative. If multiple players tie, they can decide among them who will go first; if there is disagreement that is not quickly resolved, the DM will decide for them.
In exploration scenarios, Environs act first, followed by the PCs in whichever order they choose, then the remainder of the Challenges at the end of the round. If there is disagreement among players for turn sequence that isn't quickly resolved, the DM will determine the order.
For more information, see Player's Guide Chapter 3: Scenarios.