First, do your research on the Naruto universe, especially about the Sharingan. Know its powers, limitations, and the significance in the Uchiha clan. Then, create an interesting character. It could be someone new who gets the Sharingan or an existing character with a new take on their relationship with the Sharingan. For example, if you choose Naruto to have a Sharingan - like ability, how would that change his character and his relationships with others like Sasuke?
If Naruto had a Sharingan in fanfiction, it would drastically change the story. He could quickly copy enemy jutsus like Sasuke. His combat ability would skyrocket as he could predict movements better. Also, his relationships might change. For example, his bond with Sasuke could be different as they'd both have the Sharingan. It would also make his training with Kakashi more interesting as Kakashi could teach him unique Sharingan - related techniques.
One way could be through a kekkei genkai transfer jutsu. If a powerful ninja, perhaps an old hermit with knowledge of ancient jutsus, decides to transfer Sasuke's Sharingan to Naruto for the greater good of the village. This jutsu might be risky but in the world of fanfiction, anything is possible.
In some fanfictions, it could be due to a special jutsu gone wrong. Maybe a jutsu that Kakashi was using to transfer some of his chakra to Naruto for a mission accidentally transferred his Sharingan power as well.
In some fanfictions, the Sharingan is used to explore the history of the Uchiha clan from a different perspective. For instance, a story could focus on a young Uchiha who is born with a defect in their Sharingan. Through their journey, they discover secrets about the origin of the Sharingan that were long forgotten by the clan, and this knowledge could potentially change the future of the Uchiha and the entire ninja world.
He might gain enhanced perception abilities, which would be a huge boost to his combat skills. With the Sharingan, he could predict his opponents' moves more accurately.
One possibility could be through a forbidden jutsu experiment gone wrong. Maybe some rogue ninja was trying to create a super - powered being and Naruto ended up being the subject. The Sharingan was transplanted into him, but only one eye was successful.
In some fanfictions, he might obtain it through a powerful jutsu gone wrong that somehow fuses Uchiha DNA with his own. Maybe during a battle where he's exposed to some strange chakra energy related to the Sharingan.
In some fanfics, it could be through a curse. Maybe an old, forgotten curse targets Naruto and as a result, he gains an eye bloodline. Another way is through some sort of experiment gone wrong. Scientists or rogue ninjas might be experimenting with bloodlines and Naruto accidentally gets affected.