The Help files are quite informative on this subject, but you need to be reading all the relevant ones
Intelligence is the principal stat affecting Mana levels, though you need to balance this against Wisdom since that is used in determining your succcess or failure at casting a prayer. Wisdom will not give as great a benefit to your mana level as Intelligence
Help mana :
Mana
====
Mana is the living inner power used by Priests, Wizards, Rangers,
Paladins and even some Rogues to cast their magic. Each spell that
is cast uses mana. Once mana is used up then you cannot cast a
spell. Mana will regenerate over time. The regeneration can be
sped up with rest, sleeping and meditation.
Help intelligence :
Intelligence
============
Intelligence (INT in your 'score') represents your character's mental
capacity. Intelligence is the prime attribute of the wizard. It affects:
- heavily influences the amount of mana a character gains at level
- affects the amount of mana regenerated or recovered at each tick
if a character is resting or sleeping
- the rate at which a character learns a new skill, spell, language
Help Wisdom :
Wisdom
======
Wisdom (WIS in your 'score') represents your character's wisdom and ability
to incorporate learning. Wisdom is the prime attribute of priests. It affects:
- the mana a character gains each time it levels, though intelligence
affects this much more heavily
- the percentage a character learns of a new language at each practice
- the ability to remove a trap or poison a weapon
Strength is the Prime attribute of fighters, but you wouldnt ignore it for a non-fighter.... Intelligence is pretty much the same for other mgaic using classes