This is all what you need know kennen.
this is my first guide and i hope so it's good.

Pros


+ Amazing Harraser
+ AOE Stuns
+ Heavy CC
+ Low Cds
+ Easy & Quick Movement
+ Laning: Energy User

Cons


- Relatively Squishy
- Needs to be in melee range to use Slicing Maelstrom
- Team Fights: Energy User
- Kennen is afflicted by the Law of Inverse Ninja Strength

I always use 21/9/0 or 9/21/0 mastery trees. use 9/21 if you team need more tank champ but if you team have tanks use 21/9.

Flash is an really amazing summoner spell. Any champion that doesn't have an escape mechanism must take Flash. Flash is a gap-closer if you are chasing enemies down or if you need to escape over a wall.
Ignite can always dish out that little more damamge after your enemy is in his turret range. And it's 2nd effect is helpful as well. And if you took Summoner's Wrath, that extra 5 AP will help early game.
Heal is a very good spell for Kennen. It will really help you on those 1v1 exchanges against certain champs like Malzahar (If you want to know who to get a heal against, check "What Enemies Bring to the Table"). My second choice after Ignite.

Doran´s ring give healt and ap for you.
rylai's Crystal scepter give healt and ap too
rapadon's deathcap give Veery much ap
Zhonya's Hourglass Give ap and armor, skip this if you enemy team have much ad champs.
Deadfire Grasp give ap and have nice passive.

In teamfights you have to open with your ulti, pop your Zhonya's Hourglass, and use your "e-w-e" combo! With this itembuild you'll be able to easily do that, and also beeing alive after the fight! Leave the killst to your teammates.