To become a movie story writer, you should start by honing your writing skills. Practice writing regularly, take writing courses if possible. Also, study the film industry, understand what makes a compelling story for the big screen. Network with people in the industry to get feedback and opportunities.
Becoming a movie story writer isn't easy. You need to be observant of life, have unique insights, and be able to translate those into engaging plots. Build a portfolio of your work and try to get it noticed by producers or agents.