Originally Posted by
mag07
You may get a lot of suggestions as there are a lot of addons that take care of the same things, it's a matter of preference.
For someone that hasn't used many, I'd recommend Quartz - it serves as a cast bar and dot tracker, and practically shows you important info out of the box with no need to fiddle around unless you want to. The only think that needs enabling post install is target castbar, unless you are happy using the Blizz default one under unit frames (it's not turned on by default for some strange reason)
Apart from your dots it's good to see cooldowns on your abilities in a user friendly way. Some people are fine with Blizzard's default, I personally prefer to be able to see the time in seconds - OmniCC takes care of that.
That and a boss mod, be it DBM or BigWigs or any other alternative and you're practically good to go.
As you get deeper into your role you may see the need for more info.
As a shadow priest you will be multidotting - being able to track your dots on different targets can be useful - not so much on trash mobs, as they don't live long enough to be bothered about ideal timings, but on council style fights. Again, several alternatives.
Another useful info that Blizzard's UI fails to convey in a proper way, is buffs on you. Trinket procs, enchant procs - it will be less relevant in WoD as dot snapshotting is practically gone and dot ticks will automatically inherit increased stats when applicable, however, it's still useful to see various buffs as it helps in decision making on the fly when determining ability priority. Once you are comfy with your spec, you'll start seeing the need for it.
A good unit frame addon gives you the ability to display all the above information exactly where you want it (I use Stuf, or an equally good alternative Pitbull), there are also addons dedicated to just displaying buffs and debuffs, like Raven. It's a matter of personal choice. They can be overwhelming to setup at first.
How far into customizing your ui do you want to go? How min/max your approach to playing is? You'd be surprised how many raiders massively under-perform despite clearing content because they can't see stuff on their screens.
When raiding, the ability to see what happens to other raiders and not just tunnel vision on your own role is paramount - you are a priest, you may need to dispel, off heal in those oh shit situations - for that you need to see the need for it. Blizz's default raid frames are fine, just make sure you use them. Alternatives are abundant - grid, grid2, unitframe addons come with their own setups as well. Pick and choose.
Will you have a viable healing offspec? IF yes, good unit frame setup for healing is quite important (I use Grid+Clique combo, in both specs for anything related to healing or cleansing)
It may be a good idea to start of with a UI pack - a popular one is Elve UI. There are pleanty out on WoWInterface.com, generic and class specific.
And last but not least, the addon that can practically do it all - Weak Auras(or for old school players, PowerAuras). It has a learning curve, takes a while to get used to using it, but there aren't many things you can't do with it. And there are imports from other players all over the net, that can savea begineer a lot of time, or give you ideas about how you'd like things set up.
Also, be sure you lurk around howtopriest.com. There aren't many classes that have such an excellent web resource, all in one place, as priests.