17k is probably ret spec and ret gear, right? Remember prot talents give you extra bonus on your stamina. Now put on some proper tank gear and you should clear 20k no problem.
969 is great but without the gear you'll still find pressure when there are some top-calibre dps in the group. Especially if they've just came back from a heroic raid with all the buffs still on them...
Also buy the BOE Crusader Square shoulders, they'll last until you get the Colossus one from heroic Gundrak, one of the instances you don't want to go without good DPS. I once found one selling for 20G on the AH...I bought mine for 100G.
With a half-enchanted set one should already be able to get over 20k health and well over 540 defence. Now spend some money and get the crafted epic helm and shield, you should be well prepared for the easier heroics. If you're rich, pick up the Titanium Earthguard Ring. It won't get replaced until raiding content.
Personally, I think running all the regular instances until Argent Crusade and Wyrmrest Accord both at Revered should be a good start. One for head enchant (37 stamina and 20 defence!) and the other for very good chest and feet pieces (blue ilv-200, heroic quality!)
If you're unsure about tanking, then go in as ret but pay attention to what the tanks do. Regular Halls of Lighting often see some heroic-geared tank coming back for the defence trinket. I ran it over and over, saw five of them go out to tanks (of couse I pass on it as ret), but I learnt a lot from observing what they do.
When I finally felt ready to go prot, I've had all the regular drop tank gear (other tanks didn't want them). Entered heroics fully enchanted at 24k hp unbuffed.
At the very least, try not to enter heroics with ret pieces (trinkets are exceptions, they're hard to come by). You're handicapping yourself by doing that, and it may ultimately cause you to perform badly and end your tanking desires right there.
