1.Having a passion or love for story writing is good but to transform it into a career is an uphill task.
2.Please research and get a rough idea about the earning capabilities of such a career. Tamil story writing for magazines, for children's books, for weekly supplement, as a fiction novel author or a movie story writer. Please continue with your present job and start gathering information about the above.
3.Writing a story, whatever language needs talent which will attract readers and publishers. Please start writing stories. Start a website or a blog and keep adding your stories regularly. The story should pay you.
4.You need to find out what the readers want, what the publishers like, basically you need to have a story that you can sell and make money (https://www.thecreativepenn.com/2016/12/14/writing-short-stories-that-sell/)
5.There would be many small books that will help you in this endeavor. Check this in Tamil. Check out the best Tamil story authors, read their books and understand their approach. This link has the 10 best tamil authors (http://besttoppers.com/top-10-tamil-story-authors/)
6.Shadilyan is a well known tamil novelist, know a little more about him and see if you can get ideas.(https://en.wikipedia.org/wiki/Sandilyan).
7.Start writing for the small but popular Tamil weekly magazines. Approach newspaper offices to help you start contributing for a sunday edition.
8.You would need to use many contact and means to get you first opening,bringing out your first book, getting your first publisher etc. Also try and see if you can participate in some story writing contests and see that people notice you.
9.Not to discourage you, give it a try but be prepared to have a backup plan if the writer option does not succeed (https://www.thenewsminute.com/article/seven-books-published-tamil-writer-still-sells-vadas-make-ends-meet-51902)