Don't stress if it's not going well right away, Archylus, you'll be tanking with the best of us in no time.
You are struggling in heroics because you're not yet immune to critical hits. When a boss's attack crits, it deals 200% of normal damage. These damage spikes are very difficult for healers to heal through, I'm not surprised you were kicked from heroics if you were taking crits.
To push critical hits off the attack table for heroics, you need to get your defense skill up to 535. You're at 529 now, you can get there easily by enchanting your gear.
Head: Arcanum of the Stalwart Protector - Item - World of Warcraft (from Argent Crusade reputation)
Shoulder: Lesser Inscription of the Pinnacle - Item - World of Warcraft (from Sons of Hodir reputation)
Back: Enchant Cloak - Titanweave - Spell - World of Warcraft (from an Enchanter)
Chest: Enchant Chest - Greater Defense - Spell - World of Warcraft (from an Enchanter)
Shield: Enchant Shield - Defense - Spell - World of Warcraft (from an Enchanter)
All of the above will get you to 548 Defense, which will make you uncrittable by heroic and raid bosses and increase your avoidance noticeably.
For future reference, it's important to properly gem and enchant every piece of armor in your tanking and DPS sets. It's fine to go the cheap route while soloing, but when you have a group depending on you, you need to be at the top of your game.
Enchantments by slot - WoWWiki - Your guide to the World of Warcraft lists all of the enchants and enhancing items you can get for your gear.
Oh, and grab some glyphs.
"The best way out is always through." —Robert Frost
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