I also recommend playing healer in PoTD to get your feet wet. There you'll only be facing really minor damage so it'll be easier to ease yourself into the mindset before moving onto something more serious. I also highly /highly/ recommend going WHM first. AST is the most powerful right now but WHM will help you grasp how most healing is done in this game with very little in the way of error. Once you've gotten used to that, and I'd personally do dungeons until Lv. 30, then I would recommend switching to AST. The only one I would avoid is SCH, simply because it's has the highest skill ceiling and you /will/ get frustrated if you aren't used to healing. However once you get used to it, and by that I mean you've been healing a little while, then it might be another class to try.