Karma has been one of my favorite champs - I play her as the support in a Kill lane bot. So far this works about 70% to 75% of the time. She is NOT a 0 cs support. She needs to clear minions and jungle creeps when she can. If your duo fails to get kills, then it's not as fun.
Build her like this:
Boot of Speed
Ionian Boots of Lucidity
Will of the Ancients
Aegis of the Legion
Rylai's Crystal Sceptor or Catayst into RoA or Chalice into Athene's Unholy Grail
Build her like this instead:
Amplifying tome +a ward (even if you're sharing farm, or going for a kill lane its still your job to ward)
Chalice of Harmony (she burns through mana really fast. Don't skip Chalice!)
Athene's Unholy Grail
Deathfire Grasp (or Morello's Evil tome, it comes down to personal preference)
Make sure to keep buying wards along the way. You're still a support!
That build above Maxes your CDR, and gives a substantial ammount of AP. From here you can either start building tanky or continue building AP.
Finish your build. Make sure to leave a spot open for wards. You'll probably be finishing your build last. nobody else is going to have room for wards, so make sure you buy a stack of wards whenever you go back to base.
Merc treads/Sorc Shoes/Boots of Mobility (pick based on your situation)