Ret is great! I’ve seen 4 general philosophies when it comes to gearing & talents:
1.) Mastery stacking with Execution Sentence or Righteous Verdict, focused on lining up big ability damage to vaporize priority targets
2.) Haste stacking with Zeal, focused on sustained dps, faster auto attacks, quicker off-heals and “smoother” playstyle with reduced GCD rotation gaps
3.) Versatility stacking with any of the 3 talents, focused on a “tankier” “juggernaut” playstyle, bigger auto attack damage, bigger ability damage (though not as much as Mastery stacking) and bigger heals
4.) Combination of the above, mixing in a healthy balance of Mastery, Haste and/or Vers
First step would be figuring out what type of “play style” you want to go for in a BG...do you want to be patient and line up big one-shot Templars Verdicts + Execution Sentences? Then focus on Mastery. Do you want to be in the heat of battle, swapping targets, filling in with quick off-heals, constant action? Then focus on Haste. Do you want to feel big and tanky with attacks that feel “heavier”, and occasionally dropping huge, slow heals as opposed to quick Haste heals? Then focus on Vers. Etc.
I’d encourage you to get at least some Talasites in your gear, but you definitely don’t have to stack them in every slot - try to find a balance between building up your health pool and having enough Strength & Secondaries to kill healers.
Personally, I started with an Aurora (Haste/Vers) build and ~2.9k HP (armory/out-of-BG). Played BGs a few weeks and decided I don't need as much survivability, and need more ability damage, so currently rebuilding for Feverflare (Haste/Mastery) and dropping some Talasites for +3 Strength gems, which will bring my hp closer to ~2.4k when finished. Basically, give yourself some options to play around with different stats, as once you actually get into BGs you'll find you need more/less in certain areas to match your playstyle.
Crusader definitely a must. Makes it all work. Also, people frequently overlook Sharpening/Weighted weapon stones...Weapon Damage is really important for Ret, and stones are super cheap and easy to make as far as consumables are concerned.
Hope this helps!