I'm a Hindu by religion. And I really respect my religion like I respect others. But from my childhood, I'm listening a lot of stories about Hindu Gods and Goddess which are interrelated to each other. But I'm thinking who wrote these stories, and any man/woman who first knew these stories, how did he/she get to know, because he/she was not God. So I want to know how and from where these stories came to mankind from God.
Many mythological stories we hear today have an oral tradition, that is, they were never explicitly written down anytime, but passed from generations through word of mouth.
To get some idea of "when", the Vedic age is approximately believed to be from 1700 BC - 150 BC and it is possible that the characters of Gods and the stories may have taken shape around that period.
While the oral tradition mentioned by Shyam is correct, you can consider the 18 puranas as the formal documentation of the stories of Gods. The 18 puranas were written by Sage Veda Vyasa and were written at least 5000 years ago. The 5000 years calculation comes from the Hindu calendar which sets a specific time frame for each yuga, manvantara and http://en.wikipedia.org/wiki/Kalpa_%28aeon%29. We are currently in the age of Kali or Kali Yuga and this started more than 4000 years ago.