This question is rather vague. Did you have a particular class in mind?
Your damage is dependent on the weapon you currently have equipped:
When equipping a Staff, INT and DEX are the primary attributes for your Magic Power rating.
When equipping a Gun, DEX and QUICK are your primary attributes for your Attack rating.
When equipping a Sword, STR and QUICK are your primary attributes for your Attack rating.
As for the stats…
STR – Each 18 STR you have will increase your block percentage by one, capping at 270 STR for 15% block.
CON – Increases your maximum life, so you’ll be able to take heavier hits. Note that as you level, the damage you take from an opponent of the same level decreases. Level difference is more important for survival than having heaps of CON.
INT – Increases your maximum SP so you can perform more skills and depend on Fairy Dust less. Also increases the effectiveness of healing.
DEX – Increases your chance for a critical hit. 105 DEX is needed for perfect crits. After hitting 105 DEX you deal one percent extra damage for every two points in DEX (“Overcrit”), capping at 305 DEX (For an extra 100% damage). Also, I believe it increases the effectiveness of healing.
QUICK – Increases your defense rating. After it gets high enough (Around 300 defense?), defense starts to be less effective…I don’t know about defense in much detail.
Please note that some of my information could be incorrect, especially when it comes to defense or overcrit. Please feel free to correct my mistakes.
Thanks to Zombiekilla2 for letting me know that it’s every 18 STR and not 15.