Every round, each combatant gets to do something. The combatants’ initiative checks, from highest to lowest, determine the order in which they act, from first to last.
Initiative Checks
At the start of a battle, each combatant makes a single initiative check. An initiative check is a Dexterity check. Each character applies one quarter of her Dexterity modifier to the d20 roll. The GM finds out what order characters are acting in, counting down from highest result to lowest, and each character acts in turn. On all following rounds, the characters act in the same order (unless a character takes an action that results in his or her initiative changing). If two or more combatants have the same initiative check result, the combatants who are tied go in order of Dexterity. If there is still a tie, roll a die.
Flat-Footed: At the start of a battle, before the character has had a chance to act (specifically, before the character’s first turn in the initiative order), the character is flat-footed. A character can’t take any action, including free actions, while flat-footed.
Joining a Battle
If characters enter a battle after it has begun, they roll initiative at that time and act whenever their turn comes up in the existing order.