Coupla things, Stask. First off, while I have posted a rather comprehensive guide to Retribution on our forums, tackling Protection (and tanking in general) is a bit tougher. My best recommendation is to hop on over to Elitist Jerks and review their Protadin guide. It's pretty good. Also, there is lots of general reference material on places like wowwiki for basic tanking roles and strategies.
You don't need to know every pull and boss in a zone when tanking, although it helps. It's perfectly fine if someone else knows the strats, as long as they can communicate them effectively. What you do need is a general understanding of the types of encounters you may face and how to deal with each one. A partial list may look like this:
* Single hard-hitting melee mob.
* Multiple melee mobs.
* AoE packs.
* Caster mobs.
* Mixed caster and melee mobs.
* Mob groups that require CC.
* Untauntable mobs.
* Mobs/bosses with special threat mechanics (dump threat, wipe aggro, etc.)
* Mobs/bosses with CC or fear mechanics.
* Mobs/bosses with elemental or spell attacks.
You also need a fundamental understanding of your own tanking mechanics, to wit:
* How to properly pull ranged mobs (line of sight, dead zones, silence/counterspell, etc.).
* How to pull a group of mobs.
* How to pull when CC is involved.
* Proper mob positioning (in your frontal cone, in melee range of all mobs, etc.).
* Which mob types you are and are not effective against. For example, pallies have a hard time generating threat on mobs that can't trigger blocks (ergo, no Holy Shield procs).
* How to properly utilize your cooldowns to maximize threat generation and mitigation.
* When to stun, and when not to stun.
* When to use "saver" abilities such as Blessing of Freedom, Blessing of Protection, Divine Shield, Lay on Hands, etc. Macros/addons can be used to assist with these.
Lastly, you need to understand the dynamics of your group and how to effectively use their abilities. Note that it's not always necessary for the tank to know these things as long as someone in the group does, but the tank is the natural player to look to for coordination. Even if you don't lead the group, you still need to understand things like:
* Which classes can CC, and how their CC works.
* Which classes/specs can survive aggro from a mob, and for how long. (In other words, do you need to taunt immediately or save it for a squishier party member?)
* What type of heals to expect from each different type of healer, and how that affects your tanking strategy. In particular, as a pally tank you should discourage use of Prayer of Mending, Earth Shield, and Lifebloom (the bloom, not the HoT), as these do not restore your mana. On the other hand, since you don't need to generate rage, Power Word: Shield is perfectly fine.
I've found that PuGging a run is possible if you as the tank have a good grasp of mechanics and have a competent healer and at least one competent DPS. Otherwise I'd stick to guild groups until you get a lot of practice in. You shouldn't expect to be a great tank just starting out. It took me several runs to get a good grasp of Brendore's mechanics, since my only previous tanking experience was leveling him pre-BC as Retribution. Even now I find myself learning and experimenting with things, such as weaving Seal of Vengeance/Seal of Righteousness to maximize threat generation.