Are you planning on mostly playing M+?
In that case, your one-off atonement applicators should mostly be PW:S for the tank/ST when it’s up, and Renew for ST otherwise. Generally though, the vast majority of the time I’m applying atonement it’s through radiance in keys, and PW:S will be something you’re most often putting on the tank.
Your stat prio is basically Haste > Crit > Verse > Mastery. Generally verse gets more valuable the higher the key you’re doing. So once you’re in the +20 area of keys, you’ll want to be going Haste > Verse > Crit instead.
As far as what talents you should take, there’s a few ways to spend your points. And they’re MOSTLY going to come down to preference.
If you don’t need a ton of healing and want to bring some damage to your group, the standard is to go mostly shadow-side talents and get the mindbender/void summoner talent line because it does a lot of damage.
That will look something like this:
With a few points in there being personal preference (painful punishment, the mindgames talents, Abyssal Reverie vs Twilight empowerment, etc.)
Goal here is to get mindbender out basically off cooldown, and while it’s out use Mind Blast/Shadow Word: Death to proc inescapable torment in mob packs. On boss, same thing but generally drop SW:D casts. Other than that, you’re playing with harsh discipline mostly (see below). You’ll get a ton of mana back from Mindbender uptime, and mindbender itself does a lot of damage.
Another build is to go all-in on dark reprimand/expiation. Basically gives you a lot of burst AoE healing by beefing up penance/mind blast as much as possible, so in shadow covenant you can do a ton of damage all at once - very good for high AoE damage fights like Hyrja or Storm elemental in Nokhud.
That looks something like this: https://www.wowhead.com/talent-calc/priest/discipline/DAQAEEVFEQRUVEqVKAQCBEMVUVVFVCRkQigCRA
Again, there’s some optional talents here - and I’m taking this mostly from what I see top players taking. Honest opinion though in this build is you should drop abyssal reverie for divine aegis, because it will always out-heal reverie above 20% crit.
The idea here is to build up harsh discipline (with smites, mind blast and solace), and when big AoE damage happens you radiance → shadow covenant → Schism → Penance → Mind Blast x2.
Personally what I would do if I was new to Disc would be a very PW:S heavy build that still retains a lot of the AoE healing from the second build here.
Something like this:
The idea here is to still do your AoE burst by building up harsh discipline, then Shadow Covenant → Schism → Penance → Mind Blast x2
When not dealing with AoE burst, you just build up that big penance and burn it outside of shadow covenant.
But what you gain by doing this is HUGE shields after every penance that you can put on your tank. Aegis of Wrath + Exaltation also makes Rapture a much more reliable cooldown, which is great for low level disc priests IMO.
Basically this is a good build for getting you pretty used to the standard big-penance build while having much stronger tank/emergency healing with giant PW:S casts, since every time you 7 bolt penance you’ll have a giant shield to drop on whoever you might need to funnel a ST heal on. And if that PW:S crits, you’ll be seeing huge shields, for like 200k as your gear improves.
If you have any specific questions, let me know.