Hey man, just copied + pasted this what I've written on another thread like this. Hope it helps ya (:
Just a short anwer, got no time (/Edit: I sacrificed my pre-school-time for this!!! That's bc I luv Kha'Zix and would like to see more guys plaing him ^^) :
What I like to do is building
a) Tiamat if you are ahead
b) black cleaver then - like all the time
c) finish Hydra if you are still ahead, then go for youmuus or
d) go for tank Items such as dead man's, spirit visage, randuin's, if burst mages: banshee's...
e) other choices are building LW early, upgrade/sell it later after your core (cleaver, warrior and Tiamat) and getting GA if you are fed. A nice little trick is to sell GA after the first activation of it's passive, bc then it's basically useless ^^ u don't need armor/MR as much as a DMG item such as Deaths Dance. After the 3 mins have passed you can get it again
And get the warrior enchantment, of course :)
For your abilities:
Start with W - Q -E
then max R > Q > W/E
Evolving:
really depends in your playstyle
I'd suggest evolving Q first = more DMG
You can also evolve E first = more/better mobility, especially good if you are playing toplane/sometimes mid or if you just wanna jump around like crazy ;)
Your must-have-evolutions are Q + E,
I like evolving R for the cool color (you will become red :3 ) and W for more utility.
In TFs what you wanna do is basically wait until the fight has already started, then jump/Ult in - kill the ADC/APC - juke with ult - jump out/or on the next target.
You are also able to counterjungle pretty well, just get a leash on your krugs/gromp and run to the enemy Blue/red Buff. For that i'd consider taking Q + E first rather than W + Q/E for better chasing/escaping.
but you have to watch the mini map and react to the other's pings in order to not give first blood to the enemy midlaner/adc
Sorry for eventual grammar mistakes, I'm no native speaker.
I hope I could help you a bit, good luck and have fun! :)
I also like playing mid/top Kha'Zix, and if you are interested in that, here's my build:
Mid: just no tank(y) Items except Cleaver, + get a Blade of Draktharr if you are fed
Top: just like Jungle build except Smite item ;)
Start new flask in both cases