Summoner Spells

Teleport is very good for making early plays and having pressure in the other lanes. You should utilize this and look for a TP Top or Bot after your first back. Ignite is good for matchups where you already have some kill pressure, but you should always take Teleport IMO.

Items

Corrupting Potion is probably the best starting item for Karma mid because it allows you to stay in lane for much longer if you play it right. You can opt for dorans ring or another item depending on matchup, but corrupting is highly recommended for most match ups. Since you took corrupting potion, you should be able to farm up quite a bit extra gold before your first back. If the gold works out, you should always pick up T1 boots so you can stay in range whenever you use Tether. If you backed with 1600 gold, you have just enough for a Lost Chapter and T1 boots, this is the optimal first back. Magic Pen rush is going to give you a-lot of damage fairly early, and should help you scale better in the late game, regardless of what the enemy builds. CDR boots are great on Karma for the early/mid game, but you will fall off once it hits late game, if you build these, you should sell them and exchange for Sorc Shoes once you have 4-5 items.
Night Harvester is great to help stick on enemies you are trying to tether, but luden's give much more burst, and the extra Magic Pen from Luden's scales well with your Sorc Shoes/Void Staff.

Abilities

You may need to get Shield second depending on the matchup. Always Max Q first!

Runes

If you are sure of your matchup in lane you can switch out the health for Armor or MR.

Conclusion

AP Karma Mid is a great pick if you want to deal high burst damage while staying very safe in lane. Her roaming potential is huge because her move speed and her ganks can be devastating if done right.
This is my first guide, hope it helps you :)