If you are at 535 defense yet, you can craft blue and a few epics w/ proper enchants and get to the crit immunity for heroics, and then start doing heroic after heroic and keep upgrading your gear w/ the Tier 9 items. Once you hit 540 defense, you are safe from crits vs raid bosses.
If you are inexperinced in tanking, then heroics is a good place to start anyway... warn the group if necessary.
Just took a look at you on the Armory and your gear looks pretty reasonable except for one thing that leaped out at me.....
You have almost no enchantments or enhancements of any kind on your gear.
Just doing that will improve what you have a lot.
Take a look at http://be.imba.hu/ to do a quick audit of your character and some suggestions on enchantments and enhancements you can do.
I noticed that you're exalted with Argent Crusade. So go get their head enchant. They are the ones to go to for tanks.
I also noticed that you're only Honored with Sons of Hodir. Do their dailies. They have a shoulder inscription that you'll find really useful.
Also you have blacksmithing as a profession. Finished leveling that profession. Getting 2 more gem sockets is a nice benefit.
In general, you may not have the best gear you can get. But you can make your gear the best it can be.