A number of different actions may be taken on your turn. The following are the most common.
In melee combat, you can use a standard action to grant advantage to an adjacent ally's attack or to boost their dodge AC by 2.
This may be a melee attack, a ranged attack, or other direct attempt to do damage to an opponent. See Attacking.
Most spells take a standard action to cast. These can be used at this point in combat. Regardless of what actions you have available, however, you may only ever cast one spell per round.
As a standard action, you may attempt to deprive an opponent in melee range of his weapon. To do so, make a standard attack as usual. If successful, your opponent drops the weapon in his hands. Failed disarms automatically provoke an attack of opportunity. If you fail a disarm by 10 or more points, then you drop your weapon instead. If you succeed by 10 or more points, your take the weapon from him.
Most all armed disarms are modified from unarmed combat techniques. As such, performing disarms is best when you have the Unarmed Combat proficiency, which allows you to add your Combat Proficiency bonus to your attack roll when attempting a disarm. Without proficiency, you only add your strength to your attack roll.
You can use your standard or move action to attempt to hide. To do so, you must first break line-of-sight with all of your enemies (one enemy that sees you can easily give up your location to others, though the GM may overrule this). To hide, you simply make a stealth check. Creatures looking for you will make a Perception check. Any success breaks the hide for all opponents. Likewise, any condition that calls attention to you will break your hidden condition (an attack, a spell that targets an enemy, loud noises, etc.). Hiding in this way usually requires a standard or movement action, though rogues can hide as a bonus action.
Enemies you hide from are considered Flat Footed against you. After your first attack, though, you immediately lose the hide condition. In some circumstances, it's possible for you to attempt to re-hide. If you attempt this in the same location, in an obvious location, or in a place without good cover, you roll your Stealth check at -20.
You can hold an action for a certain trigger, releasing the action later in combat. This allows you to use a standard, movement, or bonus action as a reaction instead. If you choose to ready a movement or bonus action, you may not use a standard action during your turn. One way or another, the standard action during your turn is the process of readying the reaction to be used later.
When readying an action, you hold for a specific trigger, which you need to describe specifically. When the trigger occurs, your action automatically happens before the trigger’s effects are applied. If the trigger doesn’t occur, you’ve lost your action. When your turn comes up again, you can continue to ready the same action, or let it go and attempt a new standard action.
When casting spells, you prepare most of the spell in advance minus the final incantation, including material components. If you abandon the spell, the material components are lost, though the spell slot remains unused.
When reading an action, any items you plan to use (including weapons) must be in-hand ready to go as you wait for the action's trigger.
As a standard action you may also take a second movement action, effectively doubling your movement during the round instead of acting combatively. Doing so provokes an attack of opportunity if you leave a threatened square.
Instead of attacking a creature, you may attack an object. Target objects may be attended (held or worn) or unattended. When attacking attended items roll your attack versus the target creature’s AC, as usual, but the item takes the damage instead of the creature. Unattended objects have an AC of 10 + size modifier if you use a standard action to attack them; as a full round action, you automatically hit unattended objects. Once hit, items might be damaged or destroyed, depending on how much damage they take (see Damaging Items).
Instead of attack, you can choose to only defend. In doing so, you gain +4 to your AC (dodge) until the beginning of your next turn. You do not get attacks of opportunity when taking total defense, though you can still perform movement actions that don't target other creatures.
Some objects are usable as a standard action in combat. These include using a magic device, reading a scroll, lowering a drawbridge, firing a ballistae, etc.
Some skills are free actions, some are standard actions, at the discretion of the GM. Generally, actions that involve instant knowledge are free actions (Religion, History, Linguistics, etc.), while those that require time to perform are standard actions (Searching a Room, Tying a Rope, Stealing, Pick Pocketing an item, etc.). Some might be longer than a standard action, as well (bandaging a wound, craft checks, appraising an item).
Some characters have special racial or class abilities that take standard actions. These are usually spell-like abilities or Supernatural Abilities. Unless otherwise noted, supernatural abilities and extraordinary abilities do not take an action.
Instead of attacking, you may withdraw from combat without provoking an attack of opportunity. To do so, you use your standard action to move to the closest square that is no longer threatened by an enemy with a melee attack. When you are no longer threatened, you may use your movement to escape.