http://www.simplyscripts.com and start reading screenplays of films you’ve alright watched. Start getting familiar with the structure of screenplays and certain terms.
2) Go to www.celtx.com and download celtx. Combines full-feature scriptwriting with pre-production support and also enables online collaboration. Mac, Linux and Windows. (It’s free)
3) Write everyday. Invest at least 15 minutes each day (I write everyday for 1 hour) Just start telling a story. Focus on a genre you love. Horror? Action? Sci-fi? Comedy? Just start writing…it doesn’t have to be perfect. I recommend you aim to write 2 or 3 short scripts. 5 to 10 pages long. While still studying other screenplays from simplyscripts.com, learn to stretch your imagination building a rich, complex characters and interesting scenes/situations.
4) Gather a crew (some