Broken down into rounds, and turns.
This is a concept some people struggle with, but in general each character might act for a total of six seconds within their turn, but it's the same six seconds that every other character experiences.
Initiative rolls (d20) determine the order of combat.
DM rolls once for a group of identical creatures.
In an Initiative tie scenario:
Determined by the DM as to who is surprised.
If neither side is being stealthy, they automatically notice each other.
Surprised characters can't move, or take an action on their first turn.
Dropping Prone is a free action.
Standing up costs half your movement speed.
When prone:
Move up to your speed(s). Different types of speed don't stack.
On your turn, you can take an action:
You can also take a bonus action, e.g. certain spells are a bonus action cast.
Other activity that makes sense, at the DM's discretion, otherwise known as a Free Action:
One reaction per round, such as an Opportunity Attack.
Swing your sword, fire your arrow, punch something.
One mêlée or ranged attack.
Some features allow for a second attach, such as the Fighter's Extra Attack, which allows you to make more than one attack (including Spell Attacks!)
To physically attack, you:
See Hide action for rules on Advantage and Disadvantage.
Ranges are limited, items detail their ranges.
Range attacks within five feet of the target have Disadvantage.
No more than one size larger may be grappled.
Athletics check contested by the target's Athletics or Acrobatics check, DMs decision if a monster.
Success results in 'grappled' condition.
Escape from a grapple can be attempted as an action, replaying the above contest.
You can move a grappled creature, but your speed halves.
Honestly grappling is a bit... weak, except in very specific situations.
Push something away or knock it prone.
See Prone for further rules.
A non-attack spell.
Spells have a casting time, indicating how long they take.
Spells indicate if they take an action, reaction, bonus action.
Different-ish to Spell Attacks, Spells specifically say if they're a Spell Attack.
Doubles your movement speed.
Any increases or decreases affects the additional movement.
Your movements don't provoke any Opportunity Attacks.
Any attack roll made against you until your next turn has Disadvantage if you can see the attacker.
You make Dexterity Saving Throws with Advantage.
If your speed drops to 0 (i.e. you've used it) you lose these advantages.
If you're incapacitated, you can't dodge any more.
You lend your action to help another creature.
Help is a little nebulous, see Example.
The creature you help gets advantage on appropriate ability checks, providing you can actually offer help, and it makes the check before your next turn.
You can also aid someone in attacking a creature within 5 feet of you. You might feint, distract, or otherwise help your ally.
Following the rules for Hiding, you might try to hide. See Example.
Targets attacking you whilst you're hidden have Disadvantage on their attack rolls.
If a target aims for somewhere you aren't, they automatically miss.
You have Advantage on attack rolls against a creature that can't see you. This gives up your location.
On your turn, state that you want to Ready an Action, using your Reaction.
Describe the circumstance, the DM will decide how vague you're allowed to be.
When the trigger activates, you can choose to enact your readied action, or ignore the trigger (saving your reaction).
Readying a spell requires that the spell has a casting time of 1 action.
You use your time to look for something.
DM might call for a Wisdom (Perception) or Intelligence (Investigation) check, depending on the circumstance.
Useful for interacting with something outside of a free action.
A DM might rule that swapping weapon requires an action. This is sometimes contentious and is ultimately up to the DM.
You can describe what you want to do, and a DM can decide if you're allowed to do it with an action, or in some other way.