INT and DEX are your Main Stats that you'll be putting points into.
INT increases your Spell Damage, While DEX reduces that Cast time (The little Bar)
I Recomend for Skills, You try to get SoulStrike first. It has a Very low Cast time, And is Fairly Strong on Undead monsters.
The Higher your INT, the more Damage you'll do, and the Higher your DEX, the Less time it will take to Cast your Spells.
If you wanted to level quickly, after getting Soulstrike level 10, You can head down to the underprisons (Its a Killsteal map)
Meaning that KSing is legal. To get there, Go to where you Changed your Job, and talk to the Stylist. Then, When she gives
you a List of Options, Choose to Rob her.
After that, Keep using the Soulstrike Method till' you get to a High enough level to change your job to a Wizard, or a Sage.
Edit: There are often really helpful People down in the prisons too. If you use your manners, and ask for Things like Heals, and Buffs nicely,
You'll most likely get some from a passing Priest. After you get quite a bit of Zeny, I recomend you bring Ygg Leafs down with you, In order to
Resurect a Priest, or Aco if you see one dead. They'll usually repay you with Buffs.
I'm down there with my Highpriest fairly often. And if you ask nicely for heals of a buff, I'll gladly help.