It's in the core rulebook on page 207 under Counterspelling.
If you don't have that, essentially you have to ready a standard action to counter an enemy's spell. When they cast you make a spellcraft check (DC 15+spell level) to identify the spell. You then have to cast the same spell back at them, and the 2 spells nullify each other (neither one takes effect). There are some spells that are opposites (like Light and Dark) that specifically say they counter each other, and "Dispel Magic" & "Greater Dispel Magic" can dispel nearly anything.
