This is my first guide, so bear with me. This is obviously a guide for Karma. Karma takes a bit of practice, but when you get the hang of her, she is well worth it. I consider her tier one support. She does take a lot of positioning and control to master. There are many different ways to build her, but I perfer the AP/CDR heavy Karma. You won't get an amazing K/D/A with her, but you should be positive, and win.

are self-explanatory.

Doran's Ring Buy this first, you shouldn't have mana problems, and you can heal yourself with Mantra + Heavenly Wave
Ionian Boots of Lucidity Don't go back to base until you have 900g, so you can purchase this and head back.
Elixir of Brilliance Get these whenever possible
Rabadon's Deathcap Build into this immediately.
Archangel's Staff If gold is a bit tight, get this immediately instead.
Deathfire Grasp You should have both items above by now, build this next
The rest is situational
Your gonna want to aim for items with AP & either CDR/Health/Actives/Passives
These are all viable choices:
Rylai's Crystal Scepter / Will of the Ancients / Abyssal Mask / Spirit Visage Banshee's Veil / Zhonya's Hourglass / Void Staff

Your gonna wanna prioritize Heavenly Wave
Try to even it out between Soul Shield/ Spirit Bond
Karma's ultimate ability Mantra is fairly difficult to use correctly.
It is extremely situational. So chose carefully what you want to do with it.
You team may need two heals, or it may need a AoE burst with Soul Shield. Your team may need you to focus more on damage, and CC, but that's all up to your scenario and your team.

I recommend Flash& Ghost They let you have better survivability early game, and provide mobility during end game battles. They are also great for escaping and ganking.
Other Viable Choices are:
Clairvoyance- For a more supportive summoner spell
Teleport- For more map control
Cleanse- To be more on the safe side

Pros:

Great farmer
Excellent Harass skills
Amazing support
Not as squishy as she seems

Cons:

Squishy w/o her shield
Difficult to use

Spirit Bond
Takes some time to learn

Use

Spirit Bond to get back to your lane faster
Try to save one Mantra Charge, but keep the other on CD
Try to use Spirit Bond as effectively as possible, this could mean bonding your tank, or someone in the back of your enemies group.
Heal with Heavenly Wave often
Don't always shield yourself
Try to keep at the back of your team if you know the opposing team is tempted to target you.
Blue Golem works well on her, just like any other AP champion.

Thanks for reading, (:

All in all, Karma is a great champion if you like AP support champs. Don't take her for granite, and don't underestimate her. She is difficult to kill if played correctly, and supports her team very well. She has decent damage, but is more of a support champ, not a mage. I think she is worth the IP, and you should at least try her out. She's really fun, and usually doesn't get boring with her.