New with LOTS of questions

[Enkidoh]

Welcome to Eorzea Nyro!

1. Any server other than Balmung really (which is the highest population US server). But as I play on Balmung I'm not sure - someone else might have a better idea of the situation on other servers than I do I'm sorry.
2. CNJ itself! I presume though you're meaning White Mage (WHM) which is the Job that 'upgrades' from CNJ, which requires Arcanist (ACN) to be levelled to at least level 15 to unlock , and can also take abilities from Thaumaturge (THM) - in fact, the later class is highly recommended to level to at least 36 to learn the Swiftcast ability. The usefulness of this ability for healing cannot be underestimated (it's especially useful for raising someone in a hurry during a battle - it removes the charge time of a spell and thus used with Raise it means you can raise someone immediately). So ACN and THM are the two classes recommended to level with CNJ.
3. I recall the development team stating a while ago that they intend to support the PS3 version til at least the new expansion (which is due out sometime around the middle of 2015). But I guess it also depends on how long Sony continues to support the console. I can only assume that when Sony finally drops support for the PS3, then SE will stop supporting the PS3 version of FFXIV as well. In any event, the PS3 version will probably be around for a fair while yet so don't worry.
4. The way the Armoury System is set up, you're free to level as much classes as you want (a 'Job' is something of a misnomer as it's just a piece of equipment - a 'soul crystal' - that piggybacks on a base class and acts as a stat booster while also allowing new abilities to be accessed as well as restricting what abilities you can take from other classes to just two classes). ACN and THM are the two classes that WHM can draw abilities from (as stated in the previous question), so they're the two classes worth levelling at least if you plan to become a career WHM.

Hopefully this hasn't be too confusing for you.