This is the build i usually follow when playing kha'zix. it provides massive damage and some sustain through life steal and the GA

your runes should be your standard AD runes but cooldown glyphs are also a nice option and armor pen marks are a sold choice as well

His passive is unseen threat which grants him bonus attack damage and magic damage on the next enemy champion after you have become not visible to the enemy. unseen threat lasts until consumed and has no cooldown.

the skill sequence should always be maxing Taste the fear first this provides you with high damage. Second should be void spike this ability gives you a little area of effect and heals you if you are close. third is leap this is his utility although it does deal damage it mostly helpful to get closer or to get away. Along with any champion void assault should be leveled up as soon as possible, even more so with kha'zix because of the passive of granting an evolution to one of his abilities

Taste the fear is a must for a first evolution the next two can change depending on your play style and enemy champions but this will help in any situation

Taste the fear
grants 12% more damage on isolated targets
bonus 50 range on basic attacks

void spikes
fires three spikes in a cone
marks enemies that are hit with unseen threat

leap
gains a lot of range
cooldown is reset on a kill or assist

void assault
allows you to cast it three times within 10 seconds
damage reduction while stealth by 40%

the abilities I usually evolve are Taste of Fear first depending on the enemy champions Leap second then finally void assault. if you are getting pushed hard and need extra damage then evolve void spikes instead of leap.