How to become a film writer?

Make sure your scripts are interesting and original enough to catch the directors eye. My main suggestion for you is if you are interested in script-writing then write. Write a lot! Whenever you have free time. If you feel like you have any good ideas write them down. I too am an aspiring screen writer (who also wants to get into directing) and whenever I feel like I have something that I think is clever or funny ect..I write it down. Carry a notepad with you, save it on a word document. Just start writing to get experience. If you think it's good, then save it and find people who can act it out. Unless you plan to direct it I would suggest finding someone who is good at directing and have them cover the rest. Do Not! I repeat! Do Not settle for less! Make sure that you get credited for your hard work so that you have the credentials to succeed in this competitive industry.