Faith: (Primary)
Monks by default start with the monodeist religion. However, Nightmare supports six religions. A newbie monk can choose from four of them via the conversion prayer. The other two religions are reserved for multi-classes. A character's prayers are dependant on what faith he is. See the religion page for more information.
Melee Attack: (Primary)
In my opinion, Melee Attack is the best and easiest attack skill in the game. Simply put, it is fighting without weapons. This comes in handy because you never have to find a weapon or spend money to repair one. This skill also has several different abilities.
Melee Defense: (Primary)
Melee Defense is also a good skill. Roughly seventy percent of NPCs wield no weapons. If you fight these NPCs then it will train your melee defense.
Magic Defense: (Primary)
Magic Defense is the primary means for a character to protect themself. By training this skill a player can cast shields to protect himself in battle. This skill can also be trained simultaneously with the above three skills.