I would start by getting all the craftable gear or normal dungeon gear you can get your hands on. You'll be getting some tanking experience from running those normal mode dungeons as well. Cider made a nice guide a while back on a bunch of gear options to get to the magical number of 540 defense (535 for crit immunity in heroics) - which is made a lot easier now with epic gems. You can check that out here: http://www.tankspot.com/forums/f97/4...ear-guide.html
Once you have the defense I would suggest hitting the new LFG as a tank and running as many random heroics as possible. There's a dozen excellent upgrades for you with all 4 tiers of badges (heroism, valor, conquest and triumph). Keep upgrading your gear, and while you are doing so, you're getting even more experience at tanking in those heroics.
If you really aren't confident just going straight into random heroics, gather up a group of friends and run some things with them. It's a lot less stressful (sometimes even fun!) if you happen to wipe with a bunch of friends, than a group of strangers.
Best of luck, if you need some help with anything else or I missed something then just ask!