The plot could start with Spider - Man accidentally getting transported to Thanos' home planet. There, he has to survive and find a way back to Earth while being hunted by Thanos. As he evades Thanos, he learns about the weaknesses of Thanos' power and technology. When he finally gets back to Earth, he uses this knowledge to fight Thanos and protect the planet.
In a Spider - Man vs Thanos fanfiction battle, it's tough to say who would win. Spider - Man has his agility, spider - sense, and web - slinging abilities. He can quickly dodge attacks and come up with creative ways to fight. However, Thanos is incredibly powerful, with his strength and the Infinity Stones (if he has them in the fanfic). But if Spider - Man could somehow use his intelligence to outwit Thanos, like finding a weakness in his armor or the way he uses the Stones, he might have a chance. It really depends on how the author of the fanfiction writes the encounter.
It all starts when Peter Parker gets bitten by a radioactive spider. This gives him amazing powers like super strength, the ability to climb walls, and a 'spider sense'.