I used to keep a whole guide on this page, but since I already did something similar to this for WoW Insider, I’m going just link what I have there. But, please do so with the following corrections in mind:
 At the time that was written, it wasn’t fully known how fantastic haste was going to be for prot paladins, so a lot of my stat priority descriptions are outdated. If you are interested in a “Control” tanking paradigm (that is, more active mitigation use through a much higher generation of holy power), then you’ll want to focus on haste.
If you’re not comfortable with active mitigation, then mastery is a perfectly acceptable focus instead. Avoidance is generally not a good idea, even with the buff it is getting 5.2. Passive avoidance just doesn’t have the same survivability return on investment that haste or mastery has.
Now, for reforging you still want to cap hit (7.5%) and hard-cap expertise (15%), and then dump whatever you can into haste. Personally, I prefer using Ask Mr. Robot‘s optimizer and telling him you want the PVE: Control/Haste setup. For gemming you want to use Solids in your blue and prismatic sockets, Forcefuls or Puissants in your yellow sockets, and Guardians in your red sockets (because we need a ton of expertise).
 In the hit/expertise section I wrote:
I do not recommend attempting to hard cap intentionally. Likewise, I do not recommend attempting to force more hit/expertise than you have on your gear. You can get more survivability from stamina, mastery, and avoidance.
Cripes, I really need to update that. Could not be more wrong! You definitely want hit and expertise at cap, about all else. Maximizing your active mitigation through holy power generation is paramount.
 For a glove enchant, Haste and Expertise are both excellent choices. You’ll likely find that the latter is preferable until you have much higher ilevels.
 Seal of Truth hadn’t been nerfed yet when that guide was written. As of now, you should be using Seal of Insight to tank. The healing procs are way more beneficial than the piddling threat that the nerfed Seal of Truth provides. The developers compensated for the nerf with buffs to other abilities, so our damage remained the same. There really is not value in choosing Truth over Insight.
 Likewise, I didn’t write anything about Glyph of the Battle Healer at the time, because we didn’t really think we would be using Seal of Insight to tank. However, now that we are, the glyph is a fantastic choice and (many would argue) a mandatory choice in most group content. Our Insight procs scale with Vengeance, which means that in raids we can do some worthwhile healing for melee participants on more raid fights.