You use the Ready action to 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.
You don't seem to Ready to take any particular action. You use the Ready action to set your weapon. It's a special use of the Ready action.
The common interpretation is that you by setting a weapon, you are automatically readying an Attack action... but that's certainly not spelled out. Nor is there any indication as to how long a weapon remains set... is it set until you make an attack? Until your next turn? Until the implicit Readied Attack action triggers?
FWIW I allow a "readied spear" to do double damage against anyone who charges the character that round. It would be ridiculous to be faced by two goblins and have to ready the spear against one (who then hangs back allowing the other to charge in with impunity).
I'm even considering a house rule that you don't have to ready spears for this - it is such a basic tactic for spear use that I'd automatically give spear (etc) users double damage if they hit someone who is charging them.