Hey I'm going to teach you the ins and outs of demo pvp. This is NOT a gear guide though I will provide what to reforge to you will have to look for a guide for gemming and enchants.
1st: Reforging you should reforge to mastery because it is now the best attribute for demo because of its extra damage in meta and its extra pet damage. Haste is also optimal but I prefer mastery for burst.
2nd Pets: In PvP you should use Felguard because of his stun and felstorm but you can use felhunter if you can keep all your dots up for the pet to do good damage but against melee it's useless apart from damage but at least felguard has a good stun.
3rd duels: In duels right now our doomguard is bugged to scale triple mastery. So he hits for 20-30k if you pop him with trinkets up then he'll hit 30-50k on 3.5k resil. So duels are easy unless its a really good sub rogue who vanishes on your meta and when you pop doomguard.
4th Arena: Since we can't use doomguard in arena we can't OMGWTFBBQPWN people quickly. In 2v2 go double dps such as sub\demo demo\demo Retri\demo. In 3v3 go Rogue lock druid or Rogue lock sham. In 5v5.... I don't have any advice for 5v5. So when you fight in 2v2 you should focus the healer (If its a pala do it until he bubbles then switch when bubble is over switch back OR when its a resto druid keep nuking him until tree of life then CC him and attack the dps until his tree of life runs out)
5th how to fight: When your partner attacks a target use you pets axe throw then make him felstorm him Then pop Meta, Demon soul, Immolation aura and your damage trinket. Then Hand of Gul'dan, demon leap, put immolate on him, spam shadow bolt until Molten Core procs then use that. Also try to maintain Bane of doom and corruption. Keep using hand of gul'dan on cooldown. If successful Target is dead.
6th tips: Pop a fear every so often on a melee attacker if hes focusing you even though melee don't hit that hard when you have 60k armor in metamorphosis. If the damage becomes too much use hand of Gul'dan then teleport to your circle.
7th tricks: In Blades edge arena don't set your circle on top of a stone pillar when fighting rogues set it in the middle to LoS Shadowstep. In Blades edge always set your portal on a pillar immediately.
THAT'S ALL FOLKS! Not all locks have to be aff to get to high rating .