PCs may voluntarily set their Initiative value to any positive number less than or equal to the actual Initiative rolled; in this manner, players may choose to start later than their roll would otherwise indicate. This must be done before the first entity takes their turn, however. NPCs cannot change their Initiative results in this manner unless otherwise specified.
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.
For more information, see Player's Guide Chapter 3: Scenarios.