Initiative
When a combatant enters battle, she rolls an initiative check to determine when she’ll act in each combat round relative to the other characters. An initiative check is a d20 roll to which a character adds her Dexterity modifier plus any other modifiers from feats, spells, and other effects. The result of a character’s initiative check is referred to as her initiative count. The GM determines a combat’s initiative order by organizing the characters’ initiative counts in descending order. During combat, characters act in initiative order, from highest initiative count to lowest initiative count; their relative order typically remains the same throughout the combat.
If two or more combatants have the same initiative count, the order in which they act is determined by their total initiative modifiers (the character with the highest modifier acts first). If there is still a tie, the tied characters should each roll a d20, and whoever rolls highest goes first. This final method of determining which character’s initiative order is earlier is often referred to as “rolling off.” However, if the GM allows it, characters whose initiative results are a tie might decide among themselves which character acts first based on strategies or other tactical factors.
A character rolls to determine her initiative count only once in each combat. Even if a character can’t take actions—for example, if she’s is under the effect of a hold person spell or is otherwise paralyzed—the character retains her initiative count for the duration of the encounter. The exception is when a character takes an action that results in her initiative changing (see the Ready an Action and Delay on page 249).
Any characters who enter combat after it has already begun roll initiative checks when they first enter combat. The GM then inserts them into the initiative order based on their initiative counts.
Source
Core Rulebook pg. 238