Hello!
I want to tell something about me playing Kha'Zix.
I have played a lot of bruisers at toplane, but i like to play champions with HUGE damage output rather than those bruisers like Darius.
When i first time played with kha'zix i noticed that it is very strong and it makes a lot of damage.
Many players i have played with didnt even know how much kha'zix really can make damage with full AD build with only few defensive items. This is the way i like to build Kha'Zix!

Kha'Zix is good with:
Blitzcrank
Amumu

Kha'Zix is strong against:
Darius
Teemo
Singed

Kha'Zix is weak against
Jayce
Yorick

Kha'Zix has a huge damage output so try to hit your enemy with your Q ability as much as you can earlygame.

Lvl 6:: Now you should upgrade W ability after you have learned Void Assault.
After that try to get close to your enemy and use your ultimate. Then hit enemy with your Q and W. Then cast your Q again and if enemy try to get away.. then just leap and finish him and get kill :)

Lategame: Now when teamfight begin and everyone is trying to hit eachother, you should to get close to their AD Carry and just trying to kill him. Then go away so you dont get killed. After that just go back to fight and try to get down their weakest champions with good damage.

If you are losing your lane.. dont lose your hope! Remember to Farm.
With a few kills you cant carry game, if your enemy have like 100 minion kills more that you even if he has one kill less than you. Farming is one of the most important things in this game.