i have 6300 ip and i want buy or kennen or swain or wukong...

HOW I BUY?

Depends what role you want to play. The nice thing about Swain and Kennen is that they are both solid top and mid lanes, can build will of the ancients for dual-ap comps, and have great team fight presence. Wukong is more of an aggressive top-laner and dominates early though great harass. Of the three, Wukong is probably the most aggressive, Swain is the most sustained, and Kennen is the best in team fights. I personally like Kennen the most, because Wukong falls off a bit late game and Swain is kinda countered by ignite, but all three are viable.

ty

I have all 3. My vote goes for Swain.

I find Wukong has a bit of a higher skillcap and can be daunting for new players on top, as he has no natural sustain and all his damage requires him to get close, putting him in harm's way. He's a bit snowball dependent, but once his damage gets going he can get out of hand pretty easily.

Kennen is great, especially once you hit 6, but for an AP champion his damage is a little underwhelming and his CC is circumstantial. Late game you're just a walking ult.

Swain has excellent poke early in his Torment, and can outsustain most laners post 6. He's also a great champ to learn a tough skillshot, as his Nevermove is delayed.

"SWAIN BABY! ;D"

Also, you click that golden button in the main menu towards the top. It's called the store. Then you click the champions tab and look for Swain :3

I say Kennen or Wukong

I only know of swain and wukong, so I can only advise you of those two.
Swain:
Pros: He has a snare and a slow for crowd control, he has a lifesteal ult that just so happens to be an aoe as well, his damage comes over time, which makes it hard for enemies to gauge just how close swain is to killing them.

Cons: He is -very- difficult to master, and his ult is easily countered. many will tell you that the amount of skill he demands outweighs his playability. His snare is severely delayed -and can be avoided if the player is anywhere inside the circle- while moving (assuming they are uninhibited). He is also item DEPENDANT. His damage is terrible without proper CS.

Wukong:
Pros: He's very versatile, and can range from pure AD to tank, to anywhere between. His ult is a disrupt and deals consistent damage. He has plenty of damage from his base attacks, and all of his skills are icing on top.

Cons: His W is countered (somewhat) by oracle, which is a risk run by all stealth characters. He has no built in sustain, making it hard for him to maintain lane against champions with sustain (Warwick, Shen, Swain, etc) He is difficult to master, though not terribly so because his skills can be used to farm effectively.

ty