Gem and enchant for defense. Your hp's are fine but you're too far below the defense cap to be able to reliably tank heroics without dying to those random crits. Replace your rings first of all. There are some cheap green JC rings that you should be able to find on the AH that have def on them. Try to grab some cheap +defense enchants and throw some defense gems in your chest and belt. Ideally you want 535 defense for heroics, but my guild's OT started running heroics when he was at ~520 I believe with similar hp/armor.
Yes 540 def is pretty much a must.. their are some good blues and greens that are pretty cheap that can be made for yor wrists and rings. But the titan steal garduon is in my openion one of the best starting sheilds that can be crafted by blacksmith and is deffently worth the investment.. for good tanking build i have tryed this and it is working great for me. Talent Calculator - World of Warcraft hopefully this helps you
Start working on that Wyrmrest Accord rep, you get two really high defense blues that you will probably wear until you have quite a few epic drops. As the others have said gem and enchant defense until you hit 540.
What'll happen is as you gear up you will start replacing those defense gems and enchants with stam gems and enchants. You will eventually start replacing your defense trinkets with non defense trinkets too. Ultimately you wanna be able to wear a nice setup of epics with no defense trinkets and no defense gems or enchants but it'll take awhile.
Also start saving money/mats for the +defense gun. It is pretty huge.
If you are only tanking 5mans, you do not need 540 defense. It's really early and I'm too lazy to check the exact amount, but it's probably somewhere around 535. Might not seem like a big difference at first, but at lv 80 you need a lot more defense rating per skill point. So if you are new to 80 and JUST running 5mans, that's a little extra room for you to gem/gear for stamina or threat stats. Just FYI.
535 def is uncrittable for any heroic, and you can sit there at that for as long as you want until you start raiding. 21-22k hp is probably a good starting goal to shoot for, especially if you are just starting out on the easier heroics. Chances are if you're hit points are around those numbers your armor is too. As for specs Cider posted a great threat based spec on the main page not long ago about warriors upping their dps as tanks. I've been told this is something of an advanced spec and not for everyone. Personally, I'd say just think hard on if you will actually ever use the talents you are choosing. I always considered the intervene talent useless since I don't make use of intervene ever really, but since I've leveled my healer I've gotten to see a lot of different tanking styles. Behold, some people do use it and they like it! Just don't be intimidated by a tough fight, it's nothing a 10 minute Hodir rep daily won't bang out of your repair bill.