Here, let me just quote BBB and cut to the chase:
It doesn’t sound all that hard, does it?That’s the secret. It’s not.
Go on. Read it. If you're here because you want to find out if you can tank Ragefire Chasm, or because you're wondering if you can handle Sunken Temple or Hellfire Ramparts or any of a number of other dungeons... head on over to BBB and see what he means when he tells you the Truth about Tanking: it's not all that hard.
Not to say that it's easy. Or that you won't be nervous or intimidated the first couple of times you tank. Or that you won't make mistakes. When it's all said and done, though, and you're standing over the final boss in RFC or Deadmines or Ramps, looking around, you'll realize that it was scary at times, fun at times, and overall...
... it wasn't all that hard.