White Mage is the easiest healer to learn. But if you like the Astrologan design, play that. You'll start at low enough level that you've got lots of time to learn it.

Early on its really just about focusing on keeping people's health up and learning how your abilities work. Once you're comfortable there, you can add in some DPS as time permits. Then they start giving you card abilities. The levelling dungeons are there to learn, so if yo ustart with those and gradually work up, you'll do just fine.

Just remember that you can pick a lower level dungeon deliberately if you want to start as easy as possible.