Initially just level up a character to 50, then a gatherer and / or crafter (some say level all the crafting jobs to 15 at least then select 1 to max out... to make HQ items)...gatherers sell high end materials for HQ items that crafters make to sell to players who spiritbind the items and turn them to high end materia (IV's, tho resistance materia is not as popular).
Basically you got to find a spot in that materia cycle chain, and earn gil through one or more of that chain...Mats -> Craft Item -> Spiritbind item to Materia. The high end Materia is in high demand, hard to acquire and therefore lucrative.

At least thats how I understand things... I may be mistaken as I am still leveling.
Hope it helps.