Actions
You'll need to track your actions carefully in an encounter. At the start of each turn you take in an encounter, you regain 3 actions and 1 reaction to spend that round. (Regaining your actions is described in detail here.) You can spend your actions in many different ways.
There are four types of actions: single actions, activities, reactions, and free actions.
Single actions can be completed in a very short time. They're self-contained, and their effects are generated within the span of that single action.
Activities usually take longer and require using multiple actions, which must be spent in succession. Strike is a single action, but Double Tap is an activity in which you use both the Aim and Strike actions to generate its effect.
Reactions have triggers, which must be met for you to use the reaction. You can use a reaction anytime its trigger is met, whether it's your turn or not. Outside of encounters, your use of reactions is more flexible and up to the GM. Reactions are usually triggered by other creatures or by events outside your control. Free actions don't cost you any of your actions per turn, nor do they cost your reaction. A free action with no trigger follows the same rules as a single action (except the action cost). It must be used on your turn and can't be used during another action. A free action with a trigger follows the same rules as a reaction (except the reaction cost). It can be used any time its trigger is met.
Activities
An activity might cause you to use specific actions within it. You don't have to spend additional actions to perform them—they're already factored into the activity's required actions. (See Subordinate Actions below.)
You have to spend all the actions of an activity at once to gain its effects. In an encounter, this means you must complete it during your turn. If an activity gets interrupted or disrupted in an encounter, you lose all the actions you committed to it.
Exploration and Downtime Activities
If an activity outside of an encounter is interrupted or disrupted, as described in Disrupting Actions, you usually lose the time you put in, but no additional time.
Actions with Triggers
There are only a few basic reactions and free actions that all characters can use. You're more likely to gain actions with triggers from your class, feats, and magic items.
Limitations on Triggers
This limitation of one action per trigger is per creature; more than one creature can use a reaction or free action in response to a given trigger. If multiple actions would be occurring at the same time, and it's unclear in what order they happen, the GM determines the order based on the narrative.
Other Actions
Gaining and Losing Actions
Some effects are even more restrictive. Certain abilities, instead of or in addition to changing the number of actions you can use, say specifically that you can't use reactions. The most restrictive form of reducing actions is when an effect states that you can't act: this means you can't use any actions, or even speak. When you can't act, you still regain your actions unless another effect (like the stunned condition) prevents it.
Disrupting Actions
The GM decides what effects a disruption causes beyond simply negating the effects that would've occurred from the disrupted action. For instance, a Leap disrupted midway wouldn't transport you back to the start of your jump, and a disrupted item hand off might cause the item to fall to the ground instead of staying in the hand of the creature who was trying to give it away.
Basic Actions
Actions that are used less frequently but are still available to most creatures are presented in Specialty Basic Actions. These typically have requirements that not all characters are likely to meet, such as wielding a shield, having a burrow Speed, or falling through the air.
In addition to the actions in these two sections, the actions for spellcasting can be found here, and the actions for using magic items appear here.
Delay and Ready: If you want to change when you take actions, two basic actions let you do so. Delay shifts your entire turn later in the round, and Ready lets you prepare to take one specific action when a trigger you choose is met.