Tanking problems

[Maelwys]

Btw, on lv.30 I should get the Paladin job right? So it's smart to get a Conjurer to lv.15 already?

Yep.

Ideally you'll eventually want to take CNJ to level 34 so that your Paladin can cross-class Stoneskin (and Marauder to 26 for 'Foresight', 'Bloodbath' and 'Mercy Stroke') but wait until your Tank is higher than that.
(Whenever you level up a new class, you get a major exp boost if you already have another class that's higher level...)

Paladin works quite well without cross classing any abilities at all, Stoneskin is just a very very good "Nice to have"- especially once you hit 50 and start running harder dungeons.