Where to start?:
As Kha'zix you should always start at the red buff. The health regen and the burn damage really speeds up your early clear.

How to clear the jungle?:
Camps in order after red buff:
1. Krugs: Kill the big one first with your smite. This is the fastest way to clear this camp.
2. Raptors: Use your W to hit all of the small ones, Q the big one and auto attack each of the small ones. After the auto attack W them again and they are going to die because of the red buff.
3. Wolfs: Kill the small ones first and then the big one.
4. Blue: Nothing to crazy, just kill it. You can ether smite blue or save it for the Gromp or the Scuttle Crab
5. Gromp: Again, nothing crazy, just kill it.
6. Scuttle Crab: If the crab is already taken, don't worry.

After the clear you will hit level 4. Now you should ether try to gank someone or you should back. If you chose to back pick up the Oracle Lens. Always try to use it before you gank.

Ganking with Kha'zix is pretty easy because of the level 3 power spike and your E ability. When you approach a lane save your E. If you just jump at them and they survive or flash you are done. Just simply walk to them. Only use your E if they try to escape or you or your teammate is in danger. After level 6 you can use your E early because you can follow your target with your ultimate.

First evolve: I prefer the Q evolve because your cleartime will be much faster and you can do a lot of damage with your auto attack after your Q, but it take practice to make full use of it. If you are behind you should evolve your ultimate.
Second Evolve: If you Evolved Q, evolve R, because if you had some lead you can extend it with your ultimate. You can show up in the middle of the lane next to the ADC. If have some experience with Kha'zix you will be unbeatable in 1v1-s. If you still behind evolve W, this will help out your team with the vision and slow it provides.
The last Evolve is preference based. Be sure to try out every evolve before you play ranked with Kha'zix!

You can already see the build i use most of the times but i will explain why i choose these items and all the other options as well.

Runes: I think First Strike is the best rune for Kha'zix. First Strike will help you get that early lead. I also use Conqueror against tanks and bruisers
Starting items: I think Hailblade is better, because it allows you to chase someone even if you used your E too fast and it can help you espace from a bad situation.
Mythic Options: Draktharr is my favourite it resets your passive, but if the enemy team have 3 or more tanks you should buy Eclips. Prowlers is also a good option for even more burst damage.
Boots: The mobility boots is really good if R is evolved, it lets you cross a long distance with R

Some items in the my build:
Serylda: You should buy Serylda in most of the matches, because you're not going to do much damage to tanks, but this item helps you out.
Chempunk Chainsword: In most of the matches, (especially in this meta) you need anti-healing.

Thanks for reading my thoughts on my favourite champion. Please leave your opinion about my guide in a comment, I would love read it. If you liked my guide please share it with your friends as well.