Before Mists came, I said I was going to try Guardian. Then Mists came and I levelled as Guardian, but I raided, dungeoned and PvPed as resto.
I tried reading up. I looked at Sunnier's site, but realised she is a tank who WAY outclasses me - I need more like a dummies guide to tanking, tanking for the tanking noob.
However, my Guardian spec didn't go to waste. There were no people needing agility leather gear so when we had 2 groups, I picked up all the offspec loots. And there were a lot. I went from crappy greens to 489 purples without much effort, and it made doing my dailies a whole lot easier. In fact, doing my dailies, especially Golden Lotus and Klaxxi, made me start thinking like a tank.
A week or two ago, I decided to start trying to queue random heroics as a tank. I told everyone as I entered that I hadn't tanked this before, and had to push thoughts of "Never tanked but wearing epic gear? Did she eBay her toon?" and "OMG they are going to kick me for being an idiot" because I realised I did vaguely know the bosses from a resto point of view, but Dungeon Journal is helpful. And I managed to survive to the end without too many wipes.
Now when I go and do dungeons with my friends, I no longer balk at being the tank. It's easy tanking with your friends. I can even tank for achievements! However, tanking with people who are more acquaintances than friends can still be stressful, PuGs are easier.
So I feel like I have gotten to my first tanking milestone! However, I am yet to gather the courage to queue as a tank for LFR. What I would really like to be able to do is to tank Dogs and Feng and not look like a complete idiot. I want to practice in LFR but you know how that is - nobody does it properly and I want to practice before I try it for a real deal, just for fun. But hey, I'll leave that for a little while yet. Gotta leave some goals for the new year.