Hello welcome to my ap karma build!Karma is a very underestimated champion she has a really big burst of magic damage as AOE she looks like an easy target but she can overpower most of her enemies easily!
However
she is going to be reworked (shame to riot) and that kind of sucks for me im not sure if i will update the guide when the rework will be up!

AP quints
Magic penetration marks
Magic resistance seals
Armor or cooldown reduction glyphs
Glyphs is an important thing because its like choosing between a smaller blue buff need or a survivability to melee champion ganks

This support build helps me in mid if u dont like it feel free to use an ap build but this one can be much more useful, as it gives u mana regeneration plus more income to rush down ur important items and dont depend in kills and creep kills only cd reduction for more mantras and
also ap, magic penetration and cd reduction in offense for greater burst against enemies

Start with a mana pot and an amplifying tome and head to mid.Kill minions while grabbing kills also when u return try to get both boots of speed and chalice of harmony if possible but at least buy the mana regen item.turn bots into ionian boots of lucidity and then purchase athene's unholy grail,go for either of abyssal scepter or void staff depending on the situatiuon, if ur enemy is stacking mr go for void staff else go for abyssal scepter from here get deathfire grasp ,Frozzen heart is also a very good item and grants a very good aura to kill ads (usually ads) easier, also morello's can provide the cooldown reduction u need plus ap finish with a rylai's crystal scepter.
Remember to have wards, blue buff and elixir of brilliance active at most times!
Important:against lux morgana and orianna in mid the best item to countair them early game is banner of command.why? because a promoted minion can get more shields before its death, because of that its easier for the promoted creep to reach them and that way make them victims of ur mantraed soul shield, so change the build and instead of getting rabadon's deathcap (after u purchase the banner of command get rabadon's and build normaly till rylai's crystal scepter at that point sell it and get the rylai's crystal scepter to end ur build!If in ur first recall in a game u have at least 2k gold rush needlessly large rod and boots and head of mana regeneration as fast as u can the rod will provide u a great ap to deal high damage and in early game she is dealing VERY high amounts of damage with it she can actually 1 shot creeps at 7 lvls that way.When u purchace deathfire grasp u will have enough cooldown reduction and it will not longer affect ur performance
so at this point sell ionian boots of lucidity and purchace sorcerer's shoes with alacrity enchantment to gain additional magic penetration!

Passive: Inner flame : Grants ap equal to percentage of health lost very useful.
Q: Heavenly wave, this is an aoe ability that can deal a good amount of damage on enemy champions or be useful to last hit minions.
W: Spirit bond is a very useful ability u can use to either escape or get closer to an enemy champion and kill them, grants u 10% movement speed and if used on allies grants them 10% movement speed , if used to enemies u will get 10% movement speed and they will get 10% slow, also enemies passing through it get an amount of magic damage u can use it on an emeny to kite them and get them to help u kill the minions fast this way
E:Soul shield, shields the target increased with ap doesnt decrease ur ap gained by passive
R: Mantra, this is not an ultimate ability and it doesnt get ranked up this empowers ur next ability giving it an additional effect can have up to 2 mantra's at a time recharges every 18 seconds recharge time reduced with cd reduction
Empowers from mantra: Q also heals allies targets including karma note:this decreased ap gained from inner flame
W double movement effects
E deals a massive amount of magic damage around the shielded target use it on minions near an enemy champion to hit them without coming close to them

U can use:
Flash
Ignite
Teleport
Exhaust
Avoid using
Clarity: mana problems till u get chalice of harmony so dont get it
Heal:Useless cuz u already got enough healing
Craivoyance:for supports u not a support
Smite: For jungler [check out my karma jungle build to learn about it(not updated for season 3 yet)]

Use ur shield to kill the gollem or the lizzard elders at lvl 6 and dragon at lvl 11 alone
minions are 1 hit by ur shield mantra when u got needlessly large rod.

Pros
Enemies might think u as useless so they will not think they can die from u (mostly till u get first blood :) )
Good in farming
Good survivability
Good escaping skills
High amounts of damage and aoe
Can farm dragon/blue alone

Cons
Ur team might think u as a troll
Takes skill
Squishy (shield can absorb very high amounts of damage though)

Karma can be counter to most melee ranged midders some examples are:
Gragas, Teemo, Akali, Katarina, Diana

Countairs to karma
Kassadin
Morgana [countered using promote(but is still a threat to u)]
viktor
Lux (countered using promote)
Orianna (countered using promote)
Kennen

Gragas:Gragas's a powerfull ap tanky champ he is kind of a famous midder, but he is not really the hardest enemy, the fact that his mana drops very easilly is a good thing to start with most gragas players start purchasing mana immediatelly harrass him while he will not get ap pick a kill and its ok, mostly use shields on creeps and get closer for heavenly wave sometimes plus ur auto attacks.
Teemo:Teemo's problem in mid is that he is very squishy and u will abuse that to be fed most teemo players are offencive and thats one more thing good for u only problem is his DoT (damage over time) but ur shield takes care of time
Akali,Katarina,Diana: all these champions are famous midders and melees (plus girls :D) and all have an ultimate that gets them to melee range so, try to harrass them and pick 1-2 kills till lvl 6 so u get ap when they get their ultimates dont be offencive to them like with gragas use ur shields to damage them whenever mantra is up but make sure they dont reach u
Vladimir:Uses health to deal damage thats a good thing that makes him easy to countair all u have to be aware of is his pool that will make ur mantra shield damage useless on him
Lux,Morgana,Orianna:if u get promote they are easy to counter just shield the promoted creep to make sure they get hit!With it high amount of health it wont die that easy from them and will be easier for it to reach them.
Veigar:Early game mana issued champion thats a great opportunity to harass him early game make sure not to get caught by his aoe stun and shield the low healthed creep that he is trying to last hit with his q (depending on the range he is from the creep use mantraed shield)

12/4/2012
Changed nashor's tooth to morello's evil tome
Added how to countair: section
Changed masteries to the right ones
12/5/2012
Updated the masteries to the new ones
Changed build and added additional options that work
6/1/2013
Happy new year to my mates ;)
Added kennen in "counters to karma" section
Changed runes to proper ones
15/1/2013
Added veigar in "how to counter" section
Added the rush of needlessly large or on early game in "items" section
22/1/2013
Added abyssal scepter
Updated creeping/jungling section
6/2/2013
Updated the build with sorcerer's shoes

Thanks for checking this ap karma build ;)
plz leave comments to tell me what u think about it.
Especially if u dislike plz let me know why
also if u r a karma player leave a comment to tell me about ur way of playing karma :)