ok i no everything bout kingdom hearts except 1 thing..how can riku and kairi hold a keyblade if the keyblade didnt choose them?

In Kingdom Hearts Birth by Sleep Terra let Riku hold his keyblade to see if his heart and will was strong enough to wield the keyblade, and of course it was. Kairi got the power accidentally after she was scared by some unversed, the monsters in BBS. She touched Aqua's blade and she was chosen. Aqua wanted to give her power to Sora, but Kairi got it. For go measurer to complete this Sora got his power to wield the keyblade when Ventus' Heart fused with Sora's. Sora was never chosen. Ventus' Heart gave him the power to wield the keyblade.

ok I know that riku got kis because was going to be the first one to the key blade but i don't know about I think riku made a key blade ment for thats problly why.

well... Riku was choosen at first to weild the keyblade because he had a strong and pure heart but fell in darkness and they choose sora and riku's blade is a different kind of blade (soul eater) I'm not even sure if its a keyblade... Yep it is
and kairi is the princess of heart so she can have her own keyblade because she's well the princess of heart

The answer is simple there is more than one keyblade master, like king mickey is one then sora and riku and kairi.The game just focusus on one master, sora

The keyblade chose Riku as well as Sora. I think Kairi can weild the keyblade becouse she and Sora are connected and he is the weilder of the keyblade.

Ok, for right now we don't know how Riku got his keyblade. I think it has to do with Birth by sleep which isen't out for America right now. Kairi has a keyblade because she is a princess and is connected to Sora.

I'm pretty sure it has something to do with Birth By Sleep, and I don't really know about Kairi, but Riku is a chosen master of the keyblade like Sora, but he wields a dark keyblade. I'm sure he said something about it in the first game.

Unfortunately for me, the reason as to why I know Riku does was spoiled randomly when I was reading about it somewhere long ago. Appearantly, Riku was supposed to be the Keyblade's Chosen one before Sora, but because Riku's heart chose and accepted darkness, making Sora's heart the next strongest heart for the Keyblade to choose.
Kairi's is kinda simple: she's a Princess of Heart, one with a pure heart of Light. That makes the Keyblade also see her as apart of it's chosen wielders.

*Spoilers for BBS*
Aqua accidentally performed the Inheritance Ceremony with Kairi. While Aqua was defending her Kairi reached up and touched the handle of Aqua's keyblade thus marking her for inheritance.
Terra did it for Riku on purpose though.

First of all, Sora's keyblade is the proper one, okay?
It's more powerful than the rest of them, why else would everyone call him THE keyblade master?
As we all know, Roxas can use THE keyblade too, but that's only because he's Sora's Nobody, so he's a part of Sora.

Anyway, Kairi doesn't have keyblade - she's not a master. She didn't SUMMON that keyblade, remember? If you'll recall, Riku summoned both of them and handed one to her, saying "Take it." Because no one summoned it away from her, she was able to use it. It has nothing to do with her being a princess of heart, she was just using something that wasn't hers, simple as that. She was terrible with it anyway...

RIKU could only summon a keyblade when he was emersed in darkness, remember? When he went back to normal, he tried to summon the keyblade so he could open a path out of The World That Never Was for them, but couldn't, because it was a keyblade of darkness.

Let's see. In Birth By Sleep, Terra gave Riku his Keyblade and made him his successor. Kairi got the power because she touched Aqua's Keyblade.

Terra did it on purpse, Aqua did it on accident.

Keyblade Wielders never let little kids touch your Keyblades, for they might have the power to wield one.