Why on earth are you in frost resistance gear?
Lots of your items don't even have defense on it. Granted, they do have resilience on them, but the items them selves are very badly itemized for tanking as they are PvP items.
Advice: Get some quest tank items (search on wowhead) or use Ciderhelms starter gear guide in the Guides section of this site. Do some normal instances first, to learn the encounters and get some better gear.
I haven't gemmed for defense at all, and I'm over the 540 minimum without doing any raids and only 3 heroics. I also see you have gemmed for strength, why? If you are a tank you should gem for avoidance (dodge/parry/block) or Effektive Health (stamina/armor).
As for what you should tank, I'd say stick to normal level 80 dungeons for a little while, work on your rep through that, and get some new items to replace the non-tanking items.
PS: The Icebane set is basicly just for 1 encounter in Naxx, where frost resistance is really handy. Other than that its pretty useless.
The box said: "Requires Windows 98 or better.", so I installed Linux.