Storytelling is an art, and like all art, it’s been studied to death. The better you can educate yourself before starting out, the more likely you are to succeed.
If you want to write a screenplay, there’s a great book called The Screenwriter’s Workbook by Syd Field. It will help walk you through the process of finding your story, developing characters, and writing believable dialog.
When you’re ready to start writing, download a copy of CeltX (it’s free) and get started.