What do I do when I hit 99?

[SpankWustler]

This guide was really helpful to my best friend when she came back after a few years away, and should be helpful to a new player as well.

It provides very honest appraisals and very practical information about a lot of end-game activities.

If you're subbing White Mage or Dancer on Dark Knight, though, you might need a little more help than that. I strongly recommend leveling Warrior and Samurai to 49, and I'm not sure what kind of resource to recommend that talks about game mechanics in such a general way that it could be useful for you.

My suggestion for skilling up is a bit cynical, but very pragmatic, so pursue it at your own risk: just multitask and do it while you're collecting the merits for Resolution or Last Resort or STR. You'll eventually need a lot of cruor to buy a bunch of key items from Abyssea and Voidwatch NPCs and even more Cruor to upgrade Atmacites in Voidwatch, so that's one more thing to rack up at the same time.

Become an alcoholic.

This is also solid advice.

[Mirage]

I'm not sure what missions to go do...

All of them. You could start with your nation, then go on to RotZ, then whatever you want.