To regret one's own experiences is to arrest one's own development. – Oscar Wilde
There are stories in all of us, and the old adage "write what you know" is always a good place to begin and keep coming back to. It only makes sense to steal and/or elaborate from your own experiences. Getting these stories out and onto paper is the root of screenwriting and writing.
Before I had children of my own, I avoided writing screenplay with kids as main characters. I didn't understand the complexities of the parent/child relationship, at least not from the parent's point of view. Now, however, children are an intricate part of my everyday life, so writing about them is much easier. I simply write what I know.
This isn't to say that you as you explore your screenwriting journey and grow as a screenwriter should never explore material that you don't know intimately, but again use logic as your guide. If you never watch rom-coms, why are you going to write one? If you're not into sci-fi, why are you going to set your world on another planet? But remember that knowledge is also your experience. You may never have been in the military or stepped foot in Iraq, but if you do the research – interview people who have and read everything you can get your hands on – your knowledge is the learning experience that is necessary to begin writing the screenplay.
One way I like to think about story development is by illustrating personal growth through an individual's box of knowledge. Everyone has one. Your box is simply what you know. And if you ask ten questions, you're bound to find some answers. Say you find three. Well now, your box has expanded. You know more, but you feel like you know less because you still have seven questions unanswered, and the three answers you discovered opened the door to ten more questions each. Therefore, even though you clearly have more knowledge (and a bigger box), you also have 37 more questions, and not knowing those answers is frustrating.
Story development functions precisely the same way. The root of writing a good story comes down to asking questions and fighting through the frustrations as you do the hard work to discover the answers.
The basic spine of any successful screenplay is character, objective, obstacles, and theme. A good story is about an interesting protagonist (character), who wants something badly (objective) and is having trouble achieving it (obstacles), and the story is worth writing because it illustrates some kind of universal message (theme).
But in order for successful development to occur, use a story questionnaire, explore story scenarios, and literally ask your way to uncover the answers that will guide you through your story. These questions and answers will guide you through your screenwriting and writing journey.