Evolve the following abilities in the order listed. I will give my reasons.
1) Evolve Wings. This will increase your leaps range as well as cause it to refresh on a kill or assist. What does this do for you? Well it allows your ganks to be a lot easier to execute as you can be half way in the lane in an instant. This will also help with turret diving if it does happen as immediately after the kill your Leap will be off cooldown and you can leap out of turret range. I recommend if you gank a pushed enemy to save the leap for when they flash. Always aim a little ahead of where they are so you apply the damage.
2) Evolve Claws. This will increase your damage to single targets significantly as well as increase your attack range. This will allow you to target the high priority targets that sit back away from team fights and destroy them. Also when ganking top or mid u will have a significant advantage as they will most likely be alone though minions do count and will ruin the bonus damage. The attack range will also give you a higher chance of applying your natural slow to the target as well as a Red or a Phage slow.
3) Evolve Camouflage. This will allow you to stealth 3 times instead of 2 and cause you to take less damage while stealthed and also make you look really cool. This will allow you to get to those high priority targets faster and be more precise when moving amidst a team engagement to the high priority targets and give you ur passive back every time you stealth. The ultimate by itself can be used very well while ganking to get a little bit of distance out of the bush before being seen or leaping and then when u apply your passive u can stealth again and get it back during the same skirmish.

I have been playing Kha'Zix since his release on the Beta and have had continued success with him. This guide should help you in getting set up but he does take skill to play and a little time to learn consider taking him in a coop before a real match and play him quite a few times in pubs before you try him ranked. Some of the items I will choose can be easily replaced by others depending on the scenario. For instance I will recommend bezerker greaves as they seem to work best on him but if the enemy team has a good amount of cc try Merc Treads or if they are heavy AD try ninja's tabi. Kha is a versatile character which goes along with his whole evolving scheme. Other Items that can go hand in hand are Wriggle's, get this if you arent being extremely successful the first few times you back, however if you do back with 1650 gold (after boots) get a BF sword instead of wriggles and build it into a BloodThirster as Kha does need some lifesteal so wriggles is good if your having trouble early and BT is great if you are getting fed. I would not recommend both, yes the lifesteal is great, but you are mostly using it as a tool to sustain a little more damage and not die in jungle so Bloodthirster is not a good investment if you get wriggle's. In my guide I will have you get a pickaxe after wriggle's this is also optional if you can afford the BF because you are doing good get it, this will eventually become an Infinity Edge in this scenario. However, if you do get the pickaxe and are running a little behind and the other team has a good amount of AP invest in a MAW of Mal. This item will give you more damage the lower your health as well as give you a shield. It is mostly for a good combo of survivability and damage. However If you do not need a MAW as the other team's AP is doin bad or isn't dealin much damage finish it into an IE. After IE I would recommend a giant's belt into a frozen mallet for the slow and survivability however if survivability is not an issue get a tri-force specifically phage first. This is your call I recommend phage for a little survivability as well as the slow. But say you are having trouble catching people get the zeal for the movement speed, crit, and attack speed. If you seem to be having trouble with damage or the exact opposite have so much nothing can stop you get a sheen as it will increase your damage much much more. Leaping in will give u the prock, then after u hit them you would probably q procking again, and if the fight stays stationary your spikes for the heal and damage applying another prock. Also sheen works beautifully with your ULT late game. I highly recommend Tri-Force if you want to do more damage however I find late game survivability becomes a huge issue in fights. This build will most likely be for the successful but not overly fed Khas however it still will be very expensive and most of these items are preference. Like you could get Sunfire, Warmog, FoN, Phantom Dancer, Quicksilver sash, Banshees, Atma's, etc. All of these are your preference.