The good news is, virtually all plate tank gear will have both Stamina and Strength ability scores. This means you don't need to worry about it too much. When you're gearing up, you need to look at the very first and definitive tanking statistic: Defense. Almost all tank pieces will have it, making it easy to determine at a glance what gear you should go for. The gear will have Defense Rating, which is different from Defense, or Defense skill. Needless to say, you raise your Defense to 400 to begin with, which is done by... getting hit. A lot.
Defense does three things - it reduces the chance to get crit, increases the chance to be missed, and increases the chance to Dodge, Block, and Parry. Every 25 defense skill will improve all of those by 1%. As you can see, Defense does a lot for tanks, making it the very first attribute that needs to be capped much like Hit is the most important thing for DPS. Unlike Hit, however, you can't rely on raid buffs to artificially increase your Defense. It's your job alone to make it to the Defense cap, or minimum.
The magic number is 540. This is the minimum Defense skill to maintain in order to be uncrittable by raid bosses. Because raid bosses have about 5.6% chance to crit a Level 80 player with Defense at 400, that's 5.6 x 25 or +140 more Defense skill that you'll need to get from gear. Because it takes 4.92 Defense Rating to grant 1 point of Defense at Level 80, that totals to about 689 Defense Rating.
Sounds like a lot, doesn't it? That's alright. This is one of a burgeoning tank's biggest hurdles when getting ready to raid. You'll probably need +Defense gems and enchants to hit the cap in the beginning, but once you get there it should get somewhat easier. You can also never have enough Defense because even though uncrittable status doesn't get any better, you still benefit from Dodges, Blocks, Parries, and Misses. Besides, a few bosses actually have a higher crit than the standard 5.6% so more Defense never hurts.
Dodge and Parry
Next we'll take a look at the two avoidance stats. Avoidance is good because it means you don't get hit at all. You avoid it. If you dodge an attack, you take 0 damage from it. It takes 39.35 Dodge Rating to grant a 1% increase in chance to dodge, so it's not exactly easy to raise that number. It's not something you should really gear for, it just makes items with Dodge a passable option. Agility, the Anticipation talent, as well as the required Defense should grant tanks enough Dodge to start with.
Similarly, Parry is another avoidance stat that you'll sometimes find in gear. It takes 49.18 Parry Rating to give 1% chance to parry, so it's even harder to boost that with gear. The only thing Parry has over Dodge is that it shortens the swing time of your next attack. It's a very minor thing, and while it helps threat generation a tiny, tiny bit, it's really not worth itemizing for Parry. The Deflection talent in Tier 1 of Retribution as well as Defense should give enough Parry to start with.
It should be noted that both Dodge and Parry suffer from diminishing returns. At a certain point, players will need more Dodge and Parry Rating to squeeze percentage points of dodges and parries. This makes purposely gearing for them somewhat innefectual and inferior to stacking more Defense.