This is a pretty general guide for mage arena-play. It might be a bit biased from my own experience, I haven´t tried every setup. It´s mainly for 5 vs 5 but if you are totally new you can probably learn something from this for the other brackets aswell. This guide is assuming a 2 healer 3 dps setup and it´s written because I´ve seen requests for arena-guides on the forums here. Everything about arena PvP can´t be written in a guide but the aim with this guide is to provide some pointers for the mage-class general role in arena. If you feel something is terrably wrong, feel free to say so, just do it in a constructive way.
Crowd Control:
Your main role is crowdcontrol, keeping your team alive.
Your main target to crowdcontrol is warrior, to keep mortal strike off your team as much as possible will make you win the mana-war. The warrior will become immune to sheep at times, then you have to resort to cone of cold / frostnova. Never ever start off with a polymorph in the beginning of a game, the enemy dispellers will just dispell it and your polymorph got diminishing returns on that target. It´s much better to wait until your team started dps:ing something so their dispellers have to choose between dispelling or healing, this makes their life harder. This also gives room for pulling their warrior to a nasty place for polymorph (fun when they fall for it, and they very often do :)).
Against teams with 2 healers and 3 dps (the most common setup) your goal is to keep the other teams DPS away from the person in your team that gets targeted. This involves frostnovaing, polymorphing and slowing the enemy melee DPS. Doing this out of sight from the enemy dispellers is a good thing and you should strive towards that (example of this is sheep behind a pillar). This requires good communication between you and your focused teammate and good timing on your different CC abilities and diminishing returns. Against teams with two melee DPS it´s good to try and get both in the same frostnova for example, don´t just use nova as soon as you get hit by something, you got healers on your team. In solo pvp you usually want to get away as soon as possible taking no damage at all, this is not always the case in group pvp.
Another thing worth mentioning is that if you see a warrior heading for your hamstered healer or yourself but not yet really in melee range, rather use a cone of cold then a frostnova, to have frostnova ready when someone is actually beating on you/your mate.
Against teams with four DPS classes you really need to be fast. Counterspell should be used to keep your team alive rather then interupting healers. It is for exampel a good idea to interupt a shadowbolt/fireball/mindflay/unstable affliction/lightningbolt. When this is done, proceed to sheeping another target fast. You should be able to win fairly easy if your teammates also interupt the other dps:ers. If you got improved counterspell it may be a good idea to just silence an arcane power mage and sheep him since his damage is instant and high.
To make polymorph harder to dispell, it´s good to put detect magic on your sheeps, paladin´s dispell can only dispell one debuff per cast so there is a 50% chance that the detect-magic debuff get dispelled instead of polymorph. Winters chill is also a great talent for this since it´s a magic debuff, it even stacks and paladins dispell will only remove one stack per cast. This is less useful against priests because their dispell removes two debuffs per cast. To make this even more effective, you can tell your teams paladin (if you have one) to put a judgement on the sheeps (crusader for example), it also counts as a magic debuff and with 2/2 in the stoicism talent it has a 30% chance to not get dispelled at all (zomg!).
Interupting and DPS:
When you get time, it´s time to interupt the enemy healers to give you a chance to finnish someone off. The best target to counterspell is the enemy paladin, second best is shaman, those two classes have no other way to heal then cast-time (except shamans natures swiftness which gives one instant heal every 3 minutes, and paladins holy shock which hardly is woth mentioning) spells and therefor sooner or later have to use something with cast-time. When the enemy team have a shaman it may be a good idea to use an ice-lance before counterspelling so your counterspell won´t hit a grounding-totem.
The best time for you to deal damage is after a counterspell. If you got any cooldowns such as arcane power/pom/combustion they should be used after a counterspell. Assist the other dps:ers in your team.
Otherwise just DPS when you feel you have time for it, though I wouldn´t recomend DPS:ing targets with high hitpoints when you are low on mana, enemy healers have lots of mana and not interupted. Rather save mana for CC then.
Never try to cast frostbolts with much melee dps on you (not even with 2piece t4 bonus). Your frostbolt will get kicked/pummeled and your frost-school will be interupted so that you can´t iceblock.
Specs:
You will want iceblock to function well in 5 vs 5 arena. That said, you should pick the spec that fits best with your team. If you feel you very much need on-demand burst pick arcane power/spellpower + iceblock build. If you have a rogue in the team you almost have to pick water-elemental to help catching runners so the rogue can DPS (this is a very personal belief). Water elemental is also good for CC with the extra nova and it provides extra sustained DPS.
If your battlegroup (like the one I play in) is filled with gimmick-teams with two warriors (no offence), i´d suggest a spec with blazing speed, frostbite and iceblock, even with icebarrier if it makes you feel better. It´s very effective. The warriors will give up on you since you are as slippery as an eal, this will give you better control over the fight and you can help others surviving and have an easier time counterspelling and dealing damage.
Edit: Video - Toy Soldiers 5 vs 5 (my POV)
http://www.warcraftmovies.com/movieview.php?id=48106
