Combat Skills

Combat skills are primarily used by the melee classes: Warrior, Thief, and Archers. There are a total of 6 elements to combat skills.
In order to learn the skills you will need skill points, to get skill points you must get combat skill books shown on the right.
Each book will allow you to learn a single skill and you can find the books from hunting any monster.

A player can only have a certain amount of points per level:

As said there are 6 elemental trees available. The elements are paired together so that if you learn from an element you cannot learn from the element it is paired with.
The pairs are:
Lightning - Earth
Fire - Poison
Ice - Wind

From the help built into the game client.