Confused by jobs...

So the way I see it FFXIV now has jobs (8 of them) and in order to be one of these jobs you have to have one class to 30 and one class to 15. So i'm a bit confused by a few other things. I notice there is something on a wiki page i was reading about Tertiary Classes. Classes that have some moves other jobs can use I believe. Do you have to level these up? I see that for SMN the 1st class is ACN. 2nd class is THM. and Tertiary Class is ARC. Do I need to get ARC to a certain level or is it just optional so that I could use Raging Strikes, Hawks Eye and Quelling Strikes?
My other question is When I get ACN to 30 and THM to 15 and lets say I take CNJ to 15 that gives me the ability to use SMN as well as SCH does that mean SMN and SCH will always be the same level as my ACN as in all 3 will always be the same level? 3 for the price of one?

1) It is optional, but you may want to level up the 3rd class if it has a cross-class skill you want.
2) yes, the job will always be the same level as the base class.

Thank you. How do you go about getting a job stone? Is there quests involved in unlocking the jobs? Are they hard or long if so or it is just something that unlocks and you get a pop up?

There's a quest doing that.
For me as a CNJ, I have to talk to my CNJ guilddude after I go about leveling my ACN to 15.

So for you, it may be whatever 15 level you need and your Archer guy master.