Any start advice ? Leveling etc (Leviathan server)

[Niwashi]

Will go healer once I can, not sure what the best time to get the 15 in CNJ will be, also not sure it really matters.

It doesn't really matter. Well, you'll need to reach the Lominsan Envoy quest in your MSQ to reach Gridania and its Conjurer's guild. But any time between then and when you reach level 30 as Arcanist is fine.

Favoring doing it earlier rather than later would give you earlier access to its cross-class skills, but most of them become more useful once you become a healer anyway. Being able to cast Protect on yourself while soloing as ACN could come in handy, but in dungeons the healer will do that for you.

There are a couple more milestones in the MSQ coming up soon after that envoy quest, though, and you might want to prioritize them ahead of cross-classing. You can get access to retainers (item storage and market board selling) at level 17 and mounts at level 20 in the MSQ.