As a screenwriter, you're constantly making decisions: from the foundations of character, story, and structure down to the littlest of details in each scene. Writing IS thinking, and when you're doing it right, you're making decisions while connecting each point of The Triangle: writer, material, and audience.
Mistakes, however, are sure to be made; that is par for the course and why we rewrite. But there is no greater mistake than choosing the wrong material. But how do you know if you're off target? Simple: fight the urge to work with material out of your realm.
Now obviously, no writer has lived in Middle Earth (Lord of the Rings) or A long time ago in a galaxy far, far, away... (Star Wars), and by no means am I implying that you should steer clear from sci-fi or fantasy. The "fight the urge" to write "out of your realm" is really just a warning to help you choose your story material wisely. It does NOT mean that you must have experienced the world personally. You don't need to have been a POW for it to be a realm you understand, but you would have to obtain insight of that world through research, interviews, or maybe simply your father was a POW and you've heard his stories time and time again.
To put it another way, you don't go to a history professor and preeminent scholar on the Spanish Inquisition if you're writing a NASCAR movie. But if your script does take place during the dawn of The Inquisition, talking with that professor would be a tremendous asset to help you understand the specifics of Roman Catholic Spain in 1478.
Write what you know. Oliver Stone was a soldier in Vietnam, so it only makes sense that he brought us Platoon – it was his realm. If you've been reading Marvel for ten or twenty years, you clearly are an authority in the world of comic book superheroes, so go ahead – write an original superhero screenplay. If you know a lot about zombies or vampires, write a script with mindless walking corpses or the blood sucking undead. It's pretty basic really: you are what you write - just make sure what you write really is who you are.