The most tried-and-true way to build an ideal group consists of the following three components:
- A Tank to take the damage (typically a Warrior)
- A Healer to keep him alive (the template class being a Priest)
- A Damage dealer (archetypes are Mage, Warlock and Rogue)
Each of the three roles can be fulfilled by many classes. Having each of them covered is not the only way to survive, but many people will initially try to build such a group when setting out for instance runs at high levels.