Initiative
Roll d20 + DEX modifier for turn order. Surprise prevents acting on the first turn. Rounds are ~6 seconds each.
When combat begins, every participant rolls initiative to determine the order of turns within each round.
Rolling Initiative
Each creature makes a Dexterity check: roll d20 + Dexterity modifier. Some features add proficiency bonus or other modifiers to initiative (e.g., the Champion's Remarkable Athlete, or the Alert feat which adds +5). The DM ranks all results from highest to lowest; this sequence persists for the entire combat unless a feature changes it.
Ties
When two or more creatures tie, the DM decides the order among tied DM-controlled creatures, and tied players choose among themselves. Some tables use Dexterity score as the tiebreaker (higher goes first), but this is not an official rule.
Surprise
If one side is caught off guard, those surprised creatures cannot move or take actions on their first turn, and they cannot take reactions until that first turn ends. Surprise is determined per creature, not per side — a stealthy rogue may surprise enemies while the loud barbarian does not. After the surprise turn passes, combat proceeds normally.
Round Structure
A round represents roughly 6 seconds of in-game time. On each turn within a round, a creature gets movement, one action, potentially one bonus action (if a feature grants one), one reaction (which may occur on another creature's turn), and one free object interaction.
Delaying and Readying
There is no official rule for delaying your turn in 5e. However, you can use the Ready action to prepare a specific action triggered by a perceivable event, which you execute using your reaction when the trigger occurs.