If you playing as zed you need to be very agressive from lvl 2 (LIVING SHADOW and SHADOW SLASH), from lvl 4 your combo will be : LIVING SHADOW; SHADOW SLASH; RAZOR SHURIKEN; SHADOW SLASH Ignite some auto attack and you have a kill. You need to gank a lot because zeds power in his ganks. If you gank a lot from mid you will get an early feed and you will be unstoppable. If you playing against your counter just ask for more ganks (thats not a shame, thats a shame if you won't ask for assistance and you die) but don't be passive!
DON'T PLAY ZED MID IF YOU WANT TO PLAY PASSIVE!!!
Just farm as you can (LIVING SHADOW to the ranged minions then you have to sthay by the melee minions and just SHADOW SLASH 2 times and you have all creeps from the lane.) As well you can farm more creeps out of the lane creeps there are the wraths (the enemys and the yours) if you push your lane you can take the enemy wraths but if you just push back your lane you can take the your wraths. With this farm you can overfarm your enemy easily.
Never start the teamfight because wou will die. Wait untill while you team won't start it. If your team started the teamfight your mission is just ult to the adc and kill him then you can kill the other heroes.
Masteries and Runes
With these masteries and runes you'll get 76 ad that means you'll get a realy high early burst just use it.
With these items you'll have a so nice dmg and you'll be enaugh tanky to save the teamfights.