The defence skills raise your defence by 1 point per rank. If you specialize, add that many specialized ranks to the situation (lets say you have 5 ranks in melee combat, and you specialize in defending with swords with all 5 ranks. That means, if you have a sword in hand while in combat, your bonus to your armor class is +10. Lets say your sword gets sundered though. With your sword destroyed, your armor class bonus is now +5 instead).
The defence skills, like the attack skills, require 3 skill points per rank if it is a class skill. If it isn't, then it requires 6 skill points per rank.
I am using the defence skills becuase the armor now gives damage reduction instead of an armor class bonus.