On start you need to farm. If you have poker on lane you need to be careful and wait for his CD and kill him. Don't forgot about that Kha'zix is one of the biggest mana eater.. He need to get all blue buffs if he can. Every time tell it to your jungler.

For good farm as Kha is best his W ability. At level 6 you need to evolve it. And you won't have problem with farm.

First what you need to know it that your prey must be solo. Jump on your prey and use W if you are close to it. If you are Vs. tank or offtank don't forgot to evolve your Q to better assassinating. :)