Buy a tome and a health pot then take your Q, with your runes and your tome you should have quite a high AP at the start. if there is a Jungler pull for him/her if needed and then make your way to mid. When there minions will already be there and you can move on to farming. This next chapter is how to farm.

Your Q will be your main poking ability and quite a significant farming one. If you are laning against a melee champion you should be quite safe but if you are against a ranged you need to be careful of their poking. Try to run around, depending on which champion you are laning against try to get a few last hits with basic attacks. If you are against a melee or someone who can't do much damage to you poke them with your Q and basic last hit minions. If your against a ranged or someone like Taric try to use your Q to last hit and if possible get a few pokes. You don't really want to try to basic attack minions while against a decent ranged champion because the cs you will get won't be worth the damage you take, especially while you only have 1 health pot. Try not to leave lane because if the opponent gets a significantly higher level you won't be happy. Another method of poking is this combo: Bouncing Blades, Shunpo, Sinister Steel then run back. If your thinking "But s/he will do a lot of damage to me while I run back!" S/he will do a bit but thanks to the speed boost of your W when you hit a champion you can get away swiftly, more so if they don't have boots either but nonetheless, you will get away.

http://www.mobafire.com/league-of-legends/item-purchase-planner#&item=21-27-21-105-15-22-15-103-104-1-40-9-26-34-9-106-115-21-79-106-90-6-11-4-4-65 is the item order. This build will get you a decent AD to help with your W and R and will give you a lot of AP to assist with all of your abilities. It also gives you life steal and spell vamp which are always nice to have. It also lowers the enemies' magic resist and also penetrates much of whats left after that (Thanks to the booties). Speaking of booties I will let you choose which enchantments you want. I personally would get alacrity.

Pro: You will be able to do a MASSIVE amount of damage to enemies, thus getting you a lot of kills. If your fed early game you will be able to get doubles, triples and if your good even quadras.
Pro: You have a good amount of health and you aren't too squishy so you can actually survive their attacks.
Pro: You start off with a high amount of damage and that should give you an advantage later game because you had one early game.
Con: Katarina is easy to counter. A stun and you will be dead is a common case. The Warmog's will try to stop that and will with some but champs with high damage can still manage it.
Con:

This will be a short chapter. NEVER ENGAGE WHEN ITS MORE THEN 1 PERSON. LET YOUR TEAM-MATES DO IT.

To put it short this monster of a build is the one I find best and I am sure you guys will too. Thanks for reading and I hope this guide helps you!