A new Rinnegan can be awakened by gaining Hagoromo's chakra, either by uniting the chakras of his sons, Indra and Asura, to form his own, or by getting it directly from Hagoromo himself. Black Zetsu spent ages seeking to influence Indra and Asura's reincarnations towards waking the... true Rinnegan... until they found a way through their master, Zazai, who was also one of the Five Gods of Death.
Indra's and Asura's reincarnations were each given a part of Hagoromo's body to use as evidence that they had gained enlightenment: Indra got his right eye while Asura took his left hand. Once this was done, Black Zetsu used his powers to induce a deep sleep, during which time he absorbed both sons' chakra.
As you might expect, this proved too much for even Black Zetsu's soul to bear, so he created two clones in their place: White Zetsu and White Fox. The former served as his backup while the latter was given the task of assassinating the five gods of death in order to free Black Zetsu from his duty to his master. However, before he could carry out this mission, White Fox was killed by Indra's son, who then took control of Black Zetsu's body. Using this opportunity, the new owner of Black Zetsu's body decided to take over the world instead!
No To activate the Rinnegan, both Indra and Asura chakra are necessary. Only a Uchiha who is Indra's reincarnation, and only if he is granted Asura's chakra, may awaken the Rinnegan. Only those like Madara, Sasuke, and other Indra reincarnations have the ability to activate the Rinnegan.
Can I awaken Rinnegan in Naruto? Yes, but only certain people can. Like Madara, Sasuke, and others.
No, because only a Uchiha who is Indra's reincarnation, and only if he is granted Asura's chakra, may awaken the Rinnegan.
Naruto might get the Rinnegan through transplantation. The only alternative would be to steal the Rinnegan's eyes from Uchiha Madara, who has already awoken it. On the other hand, just merging uchiha cells into his body could not activate a Rinnegan. This means that either Naruto himself or another Uchiha must donate their eyes.
No, but he can learn about it through books and conversations with others who know more about it than him.
No, but he learned about it through newspapers and magazines. Also, there are people who know this secret that don't talk about it.
Because he was Indra's transmigrant, Madara was able to awaken the Rinnegan. During his struggle with Ashura's transmigrant, Hashirama Senju, he chewed off a chunk of Hashirama's flesh. This allowed Madara to transmigrate into him, giving him access to Hashirama's powers. Then, using these powers, he fought and defeated Ashura.
After this, Madara used his new powers to search for any remaining Usui/Rinnegan users in order to absorb their abilities too. He found Hashirama Senju and killed him again, this time without chewing anything off of his body. Using this second chance at life, Madara decided to use Hashirama's powers to create a world where both he and Hashirama could live in. The result of this effort is the afterlife world of Ryūgamine.
Now that he has absorbed all eight powers, Madara plans to invade Earth using its inhabitants as vessels. However, since not everyone can become an Indra, only those with the strongest wills will be chosen by Madara. Those who are chosen will be given a year to train before being taken to Ryūgamine by Madara himself. There, they will be forced to fight each other until only one remains. This person will become the next God of Death and take over Madara's mission on Earth.