Let's say I'm holding a spear (or halberd, or urgrosh, or any other weapon that can be braced) and ready an action to attack the next enemy that comes within reach. If someone charges me, triggering my readied action, will I deal double damage on a hit? Or must I specifically ready to attack the next charging foe, wasting my readied action if they decide to move into reach without charging?
Readying a Weapon against a Charge
You can ready certain piercing weapons, setting them to receive charges. A readied weapon of this type deals double damage if you score a hit with it against a charging character.
Notice how this does not describe any particular readying process: no special triggers or requirements, simply a conditional damage bonus. It refers to “a readied weapon of this type,” without saying it needs some kind of special readying. In short, what this rule is saying is if you ready an attack, and score a hit with that readied attack using one of these weapons against a charging creature, you deal double damage.
However, please note that readied actions are an extremely niche option; they are almost never the best course of action. Going before your opponent (read: going more than your opponent) is just about the largest combat advantage available in the game. Giving that up for a single attack, even a double-damage attack, is almost never a good choice.