Jax wants to go 21-9-0, picking up Attack Speed, Attack Damage and Ability Power (though you only want to take flat AP and Scaling AP, not +6% of your bonus AP as you will not be building AP on Jax most of the time).
You also want Spell Weaving and take +2% increased damage in return for taking an additional +1% damage.
In Defense you want 1 point in Block (2 in the jungle mastery if you are playing in the jungle and 1 in the advanced version of it), +5% bonus MR and Armor, +2 Health/5 seconds, +36 health and +3% maximum health.
For runes, go Attack Speed quintessences, Attack Damage marks, Armor or Scaling Health seals and Scaling Magic Resist glyphs (though you can use flat MR ones if playing against Ryze or other heavy AP champs).