Prior to Naruto being Kyuubi's jinchuuriki, Naruto's mom, Uzumaki Kushina, was Kyuubi's second jinchuuriki. When Naruto was born the seal grew very weak and Madara managed to break the seal and use the Sharingan to control Kyuubi's mind and force him to attack Konoha. Nobody really sent him, he was freed from his seal but then the Yondaime sealed him into Naruto.
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The Kyuubi attacked Konoha twelve years ago and naruto`s dad [the fourth hokage] sealed it in naruto`s body. But afterthat the fourth hokage lost all his chakra and nobody could heal him and he died.
The Fourth hokage died protecting Konoha from the nine tailed fox. The jutsu used to seal the Kyuubi in Naruto killed him in the process.
Tobi killed her almost as soon as Naruto was born in order to release the Kyuubi into Konoha. Then Namikaze Minato sealed half of its chakra inside of his newborn son, and the show starts.
Sora is a pseudo jinchuuriki. After Kyuubi's attack on Konoha (Madara forced him to using his Mangekyou Sharingan) Kazuma (Sora's dad, one of the 12 Guardian Shinobi who went rouge) sealed the remnants of Kyuubi's chakra into his own son. At first Sora couldn't handle the power and went insane. After that everyone at the Fire Temple treated him with contempt, like how Naruto was treated.
Chapter 299 was the first time Naruto went kyuubi.
Nope. Kyuubi is the strongest. The Bijuu's level of power comes from the amount of tails they have, 1 being the weakest and 9 being the strongest. Shukaku is the weakest, as he is the Ichibi (1-Tail). Kyuubi literally translates as "9-Tails" which is what the English version calls him.