Your Three Actions
Combat runs on 3 actions and 1 reaction per round, refreshed at the start of each of your turns. There is no move-then-attack structure to obey. The full rules are in @Missing file and @Missing file.
Spending them
- Single actions cost 1: Strike, Stride, Step, Interact, @Missing file, Seek, Demoralize.
- Activities cost 2 or 3: most spells are 2 actions to cast, a few are 3.
- Reactions have a trigger and fire on somebody else's turn: Shield Block, a readied action.
- Free actions cost nothing: Release, Delay, and anything an ability hands you free.
The sheet prints actionsPerTurn, and it is not always 3 — quickened adds one, slowed and stunned take them away (the larger of the two applies). Read it at the top of your turn before you plan.
The multiple attack penalty
Your second attack on a turn takes −5, the third and beyond −10 (−4 and −8 with an agile weapon). Everything with the attack trait counts, including Grapple, Shove, Trip, and spell attack rolls, and it resets at the end of your turn.
This single rule is why the third Strike is usually the worst thing you can do with an action. Better third actions: Raise a Shield, Step out of a flank, Demoralize, @Missing file on the thing that just hit you, or @Missing file so the next ally rolls better.
Attacking
Roll mainHandAttackBonus against the target's AC, read the degree, apply damage from mainHandDamage. A critical hit doubles the damage. Flanking — you and an ally on opposite sides — gives the target the off-guard condition, a flat −2 to their AC.
Position matters
Cover is a circumstance bonus to AC: lesser +1, standard +2, greater +4. Step moves you 5 feet without provoking. Not every creature has a reaction to punish movement, and @Missing file is how you find out which ones do.
Going down
At 0 Hit Points you fall unconscious with dying 1 — dying 2 if the blow was a crit. At the start of each of your turns you roll a flat check against DC 10 + your dying value to claw back down. You die at dying 4. Coming back up leaves you wounded, which makes the next fall worse. @Missing file and @Missing file have the exact ladder.
Which is the real argument for @Missing file, a shield, and a second look at that third Strike.