All right. Since I have noticed that most shadow priests that joins our guild perform bad in the beginning and I have to hand out alot of tips I thought: Why not post a guide here so too help some people that need it
Playing the shadow priest isn't about following a rotation, it is about following a prio order for which spell to cast. The order goes like this:
1. Vampiric touch - this is the dot with the highest damage and it also regnerates mana to 10 raid members. How great isn't that?
2. Shadow word: pain - Just a normal dot with some nice dmg
3. Devouring plague - this dot is pretty mana expensive and can be left out at some points when it is really tough with the mana but really, it is a great dot since it also heals you.
4. Mind blast - a plain damage spell which should be cast whenever it is off cd.
5. Shadow word: death - You should also keep this one on cd BUT you have to be careful and observe your health before casting this one.
6. Mind flay - This is the spell you cast when everything else is done!
Making a long story short: Keep the dots up, keep MB and SW: D on cd and then, Melt some brains with your mind flay
This is the max damage "rotation" or whatever you want to call it but it is not very mana effecient and you may have to keep that in mind some times but usually it will be all right with your fiend and dispersion.
Besides casting theese spells you should also keep up vampiric embrace. It is a spell that heals whenever you do damage.
AoE: Since WotLK arrived shadow priests can put out massive aoe damage if played correctly. In Naxxramas for example, most groups can be aoe'd since they are pretty big. My thumb rule is that if you have 4 mobs in a group - AOE and keep the dots up on at least one of them!
If there are alot of low health mobs, for example the small spiders in Naxxramas, just spam the mindsear and you will crush all the others in dmg done
Also use haste trinkets or similar things at aoe since they really boost your damage with mind sear.
I will link the most common and in my opinion the best shadow priest specc in this moment.
Talent Calculator - World of Warcraft
the 13/0/57.
You get all the most neccesary things. Some people wonder "Why don't you specc Shadow affinity for reduced threat?"
The simple answer to that is that most shadow priests dont build that much threat, it is only in some extreme cases and therefore it isn't neccesary.
Theese are all the things that I can think off right now. I truely hope that you have enjoyed reading my guide and that you have learned something from it.
Thank you for your time.
Best regards
Slarviz